What is an API? Application programming interface explained

Application programming interfaces (APIs) are everywhere, even if you never see them. They’re the hidden connections that allow your tools to talk to each other, your workflows to run smoothly, and your business to scale. 

If you’re a project manager or operations leader, you don’t need to know the low-level code of an API, but you do need to understand what APIs are, how they work, and how they impact planning and productivity.

In 2024, 74% of teams were API-first, which rose from 66% in 2023. That’s not surprising. In a world of cloud platforms and distributed work, APIs are now strategic infrastructure.

You might be wondering: How does the tiny but mighty API make such an impact? Let’s review the basics of APIs and how they integrate applications.

When APIs are designed well, you don’t notice them; you just see information showing up where it should and workflows that “just work.”

In the words of an astute Redditor, “You don’t need to know what program you are connecting to, that’s the point of an API. It might be a guy in China typing on a keyboard for all you know. An API is just a contract. You give me this, I’ll give you that, you don’t need to know the details of how I got it.”

How does an API work?

Put simply, an API acts like a virtual middleman, relaying information between a client (like a mobile app) and a server. When you request data, such as weather updates, the API call goes to the server, retrieves the relevant data, and sends back an API response.

APIs are fixed, custom-built connections that developers hardcode, whereas Model Context Protocol (MCP) is a universal connector designed for AI-native interactions. One requires developers’ manual effort, and the other is designed for LLMs to call APIs without human setup.

At a high level:

1. A client application (like a mobile app) makes an API request for information.
2. An API server receives the request, processes it, and retrieves the relevant data.
3. The server sends back an API response, often in a structured format.

This request–response cycle ensures that data is always pulled consistently and predictably. These exchanges are designed to be lightweight, repeatable, and stateless. That means the API doesn’t save client data between multiple requests; it simply returns exactly what was asked for.

Client-server model 

APIs follow the client–server model. The client (like an app or web service) calls the server (where the data lives), and the server responds with exactly the requested data. This model ensures that client applications stay lightweight while servers handle the heavy lifting.

Endpoints and methods

Each API defines endpoints, which are specific URLs that represent pieces of data or functionality. For example, /projects might list all projects, while /projects/123 retrieves one. APIs are stateless, so they don’t save client data between multiple requests. This makes each call independent, and the APIs scalable.

Real-world API examples

Even if you have no idea what an API is, you are using them almost daily. When you check the weather on your phone, the app makes an API call to a weather service’s API endpoint. The API retrieves the relevant data (temperature, forecast) and returns it. Thanks to that invisible exchange, your app displays the results instantly. 

Here are some other real-world examples of APIs in action: 

• Sharing flight information between airlines and travel sites
• Using Google Maps in a rideshare app
• Building chatbots in a messaging service
• Embedding YouTube videos into a webpage
• Automating workflows between B2B software tools

And they’re not just used in apps. In a few cases, APIs have been at the forefront of innovation in the sciences. Take, for example, research in genomics: Without global collaboration and quick access to data, research remained siloed and progress was slow. Now, APIs like Google Genomics allow researchers to easily analyze vast quantities of genetic data, helping them discover new treatments and learn more about how genetic conditions develop.

Other examples of APIs include The New York Times, which allows you to analyze its database of thousands of articles, and even NASA has an open API that’s chock-full of satellite imagery and constellation data — all available for public use.

Types of APIs

The way an API is designed and shared changes who can use it, how secure it is, and what kind of workflows it enables. For leaders, knowing the different types of APIs is less about the code and more about planning: Which APIs make sense for external partners? Which should stay inside your organization? Which can scale with your strategy?

Here are the four main types:

Open APIs (public)

An open API, sometimes called a public API, is an interface that’s made available for anyone outside an organization to use, usually with published documentation. Open APIs allow external developers, partners, or even customers to connect to a platform and exchange data without needing private access agreements.

Examples of open APIs:

• Google Maps’ API powers mapping and navigation inside thousands of apps (such as Uber, Lyft, etc.).
• Twitter’s API allows external tools to pull tweets, analytics, and engagement data.
• Spotify’s API lets developers embed playlists or integrate music into other platforms.

Partner APIs

A partner API is shared only with selected business partners, not the general public. It is designed for secure collaboration between organizations. It is typically used to ensure sensitive data can be shared without the risk of being leaked to the public. 

Examples of partner APIs:

• UPS/FedEx allows retailers to integrate shipping APIs to generate labels and track deliveries, which are available only to business partners.
• Fintech apps like Venmo use partner APIs to connect securely with banks.
• Salesforce provides APIs for certified partners to build secure integrations into its platform.

Private APIs (internal)

A private API (sometimes called an internal API) is designed to be used only inside an organization. They connect internal systems and software components to help teams keep data consistent across departments without exposing sensitive data outside the organization.

An example of a partner API:

• Amazon’s REST APIs in API Gateway are only accessible from within an Amazon virtual private cloud (VPC).

Composite APIs

A composite API bundles multiple API requests into a single call and returns a unified response. This can reduce the number of back-and-forth requests (making apps faster) and simplify workflows that need data from different software systems. 

Examples of composite APIs:

• Microsoft’s Graph supports batch requests, which allow you to pull user information, calendar data, and emails from Office 365 in one call.
• Salesforce provides a composite API that lets developers bundle multiple operations (create, update, query) into one request.

API Protocols: REST, SOAP, GraphQL

If APIs are the “bridges” that connect software systems, then API protocols are the traffic rules that decide how information moves across them. Different protocols define how requests are made, how responses look, and how reliably the exchange happens. Three of the most important in modern business are REST, SOAP, and GraphQL.

SOAP (Simple Object Access Protocol) 

SOAP is an older, XML-based protocol designed for highly structured interactions. It supports features like built-in security, remote procedure calls, and formal contracts defined in an API specification. 

While less flexible than REST, SOAP’s reliability makes it valuable in environments where compliance and precision can’t be compromised.

REST (Representational State Transfer)

REST is the most common standard for web services and network-based software architectures today. REST APIs use lightweight messages, often in JavaScript Object Notation (JSON), to exchange data quickly and flexibly. They’re popular because they’re easy to work with and scale well across cloud systems. 

If you’ve used integrations powered by an API provider like Google or Salesforce, you were likely relying on REST.

GraphQL

GraphQL is the newest of the three. It’s a query language that gives clients more control over what data they receive. Instead of pulling a whole dataset, GraphQL lets you ask for exactly the fields you need — nothing more, nothing less. That efficiency is why it’s popular for modern apps that combine data from multiple systems, such as dashboards or mobile apps. 

It also pairs well with secure tokens like JSON Web Tokens to manage access.

Benefits of APIs

For organizations managing multiple tools and complex workflows, the right API strategy can make the difference between siloed systems and a well-connected digital ecosystem.

• Interoperability: APIs make it easier for tools from different vendors to work together.
• Faster time to market: By reusing existing APIs, teams can launch new products, features, or integrations much more quickly.
• Improved customer experience: APIs allow organizations to surface real-time data (e.g., order tracking, shipment status) directly to customers.
• Scalability: APIs enable modular, microservices-based systems that scale individual components without disrupting the whole.
• Ecosystem building: Companies can create ecosystems of partners and developers who extend their platform (e.g., app marketplaces powered by APIs).
• Data consistency: APIs ensure a “single source of truth” by synchronizing data across systems automatically.
• Agility: Teams can swap out or upgrade systems more easily because APIs standardize the way tools communicate.

Wrike’s API 

Wrike’s API is designed with flexible API design principles so organizations can connect Wrike seamlessly with the rest of their ecosystem. Whether you’re automating routine tasks, syncing data across systems, or building custom workflows, it gives you the control to shape how work flows in your business.

Through the API documentation available in the Developer Portal, teams of all technical levels can explore how to:

• Configure apps with OAuth for secure API access
• Manage tokens that safeguard sensitive data
• Register multiple apps without limits
• Track usage in the API App Activity panel
• Monitor integrations and audit permissions

And with Wrike’s support for the Model Context Protocol (MCP) Server, AI assistants like Microsoft Copilot and Claude can tap into your Wrike data securely and bring the power of AI directly into your day-to-day operations.