API vs SDK: What's the Difference? (With Examples)

The two terms get confused because they overlap: most SDKs contain APIs, and both sit at the center of modern app development. This guide defines each, compares them side by side, and shows when to use which, including a worked example of how the choice plays out when you add chat to an app.

What is an API?

An API is a set of instructions and protocols that allows software systems to exchange data and functionality. Developers use APIs to connect an app to external services, pull data from other systems, or expose their own features for third-party software to use.

MuleSoft's well-known restaurant analogy captures the idea in one image: the API is the waiter who carries your order to the kitchen and returns with your food. You never deal with the kitchen directly; you interact with a defined, reliable messenger that translates your request and delivers the response.

APIs also save developers from writing custom code for every capability. As IBM notes, APIs allow companies to open up their applications' data and functionality to third-party developers. A food delivery app, for example, can verify a user's phone number, display live locations, and process payments without leaving the platform, each through a dedicated API.

What is an SDK?

An SDK (Software Development Kit) is a bundle of development tools used to build applications for a specific platform, operating system, or service. Where an API defines how software communicates, an SDK supplies the building blocks: libraries, documentation, code samples, debugging utilities, and usually one or more APIs.

According to Red Hat, a typical SDK contains a compiler, a debugger, and APIs, and may also include:

• Documentation
• Libraries
• Editors
• Runtime and development environments
• Testing and analysis tools
• Drivers
• Network protocols

To extend the restaurant analogy: if the API is the waiter, the SDK is the fully equipped kitchen, with the ingredients, utensils, and recipes a chef needs to produce the dish. It provides everything required to create the intended application, not just the channel to request it.

SDKs matter most when you develop for a specific platform. Apple provides iOS SDKs so developers can build applications specifically for iOS, and Google does the same for Android. A good SDK is easy to adopt, documents its code thoroughly, and adds functionality to an existing app rather than complexity.

How we compared APIs and SDKs

We compared the two on the six aspects in the table below: what each one is, what it contains, its primary purpose, its footprint in your app, typical examples, and when to use it. The definitions draw on published explainers from Red Hat, IBM, and MuleSoft, and social.plus appears in the examples because it ships both a Chat API and a Chat SDK, which makes the contrast concrete. Disclosure: social.plus publishes this blog and builds community and chat infrastructure, so treat its inclusion accordingly.

API vs SDK: the key differences

The core difference is this: an API is an interface for communication between software, while an SDK is a toolkit for building software, and SDKs typically include APIs. Put simply, an API tells your code what it can ask another system to do; an SDK hands you ready-made code to do it.

Three points worth remembering:

• APIs facilitate communication and integration between software systems.
• SDKs provide the foundation to build an application for a specific platform.
• SDKs contain APIs; APIs do not contain SDKs.

An API inside an SDK stays lightweight and specialized for its function. The SDK around it carries the wider collection of utilities you need to build new applications or add new capabilities to existing ones.

When to use an API vs an SDK

Use an API directly when you need fine-grained control, server-side logic, or an integration that must fit an existing architecture. Use an SDK when you want to ship platform-specific functionality quickly, with pre-built and tested components doing the heavy lifting.

Adding chat to an app shows how the choice plays out in practice. A brand that wants messaging wired into its own backend, for example to route conversations through existing support systems or apply custom business logic, can call the social.plus Chat REST API directly. The API approach keeps full control over data flow and server behavior, at the cost of building more of the experience in-house.

A team that wants in-app messaging live in days rather than months can integrate the social.plus Chat SDK instead and drop in UIKit components for ready-made messaging interfaces. The SDK manages real-time message delivery, UIKit supplies the interface layer, and engineering time goes into the product rather than messaging plumbing. The same pattern applies across the wider social.plus product suite: APIs for control, SDKs with UIKit for speed, both built on the same infrastructure.

The stakes of this decision keep rising because in-app engagement is where apps compete. According to Sensor Tower's State of Mobile 2026 report (2026), users spent 5.3 trillion hours in mobile apps in 2025, roughly 3.6 hours per day per user. Social features such as chat, communities, and feeds give users reasons to spend more of that time inside your app, and the API-or-SDK decision determines how quickly and how deeply you can deliver them.

Frequently asked questions

Is an API part of an SDK?

Usually, yes. Most SDKs include one or more APIs alongside libraries, documentation, and development tools, because the software you build with an SDK still needs a defined way to communicate with the underlying service. The relationship only runs one way: SDKs contain APIs, but an API never contains an SDK.

Can you use an API without an SDK?

Yes. A web API, such as a REST API, can be called directly over HTTP from any programming language without installing anything. Teams take this route when they want minimal dependencies or work in a stack the SDK does not cover, accepting that they must write their own authentication, error handling, and retry logic.

Is REST an API or an SDK?

REST is neither; it is an architectural style for designing APIs. A REST API is an API that follows REST conventions, using standard HTTP methods to expose resources. An SDK can include a client library that wraps a REST API in convenient, language-native functions.

Which should I use for adding chat to my app?

For most apps, an SDK is the faster path: the social.plus Chat SDK with UIKit provides pre-built messaging logic and interface components, so teams can ship chat without building real-time infrastructure themselves. Choose the Chat API directly when you need custom server-side control or a headless integration. Many teams combine both, using the SDK in the client and the API on the backend.

Do SDKs slow down your app?

A well-built SDK has minimal performance impact, but every SDK adds some size to your app binary. Modular SDKs limit this by letting you import only the features you use. Before committing, review an SDK's size, update cadence, and documentation, since a poorly maintained SDK is a bigger risk than a large one.

Choose the layer that fits your architecture

APIs and SDKs are not competitors; they are layers of the same infrastructure, and strong products use both. The right question is not which one is better, but which entry point fits your team, your timeline, and your architecture.

social.plus provides both layers for social infrastructure: APIs when brands need control, and SDKs with UIKit when they need speed. If you are weighing how to add chat, communities, or feeds to your app, review our pricing or talk to our team for a demo of what the SDKs and APIs can do in your stack.