Stop treating them as rivals. An API is a mechanism that lets two software components communicate using agreed protocols; an SDK is a set of platform-specific building tools, such as libraries, debuggers and code samples, that often includes APIs. Use an API to connect your software to a service, and install an SDK when you're building for that platform.
Vendors drop both terms into sales calls and help pages as if everyone knows the difference, and it's easy to nod along. This page explains the two in plain English, using AWS's own comparison page, then works through which one matters for the job in front of you.
What an API does in plain terms
AWS defines an API as a mechanism that lets two software components communicate using predetermined protocols. Think of it as an agreed way of asking another piece of software for something and getting a predictable answer back.
APIs let you integrate predeveloped functionality in your own code, connecting your software with preexisting modules and third-party services. When your website shows a map or your CRM pulls in sign-ups from a form, an API is usually doing the talking behind the scenes.
AWS also points out that an API abstracts the complexity of exchanging data and helps ensure data integrity. In plain terms, you don't need to know how the other system works inside; you only need to ask in the format it expects.
For a marketer, that's the practical meaning of the phrase 'integrates with' on a vendor's website. Behind it there's usually an API on at least one side, letting two products swap data without anyone copying and pasting between them.
What comes inside an SDK
AWS defines an SDK as a set of platform-specific building tools like debuggers, compilers and libraries, and it often bundles code samples and documentation too. You install an SDK on your computer before developing an application, which is the clearest practical difference: an API is something your code calls, an SDK is something a developer sets up.
SDKs exist for mobile development on Android and iOS, for cloud platforms, and for specific languages or frameworks. AWS's own example is its SDK for Python, Boto3, used to integrate Python applications with AWS services.
That bundling is the point. Rather than hunting separately for a library, a debugger and examples that work together, a developer gets a set built for the platform they're targeting, with the documentation to match.
Layers, not rivals
This is where the usual comparison goes astray. AWS says SDKs may include APIs among several other resources for the platform they support, and that you can use an SDK to create new APIs. So the two aren't competing options. An SDK is often the kit that makes an API easier to use from a particular language, and an API is often what an SDK is built to talk to.
That's why a vendor can honestly say they offer both. The API is the agreed way in; the SDK is the set of tools they give developers to use it comfortably on one platform.
Picture a developer adding a cloud service to a Python app. They could write every request to the service's API by hand, formatting each one and handling each reply. Or they could install Boto3, AWS's SDK for Python, and work with its libraries instead. Either way the API is doing the talking; the SDK just saves them building the plumbing themselves.
SDK and API in one table
Both are listed alphabetically, using AWS's definitions.
| Tool | Plain definition | Who works with it |
|---|---|---|
| API | Interface letting software components talk | Connecting your software to other services |
| SDK | Toolkit of libraries, compilers, debuggers | Developers building for one platform or language |
API
Best for: Connecting software to other services; AWS defines an API as a mechanism that lets two software components communicate using predetermined protocols.
SDK
Best for: Building for a specific platform; AWS defines an SDK as a set of platform-specific building tools like debuggers, compilers and libraries.
If you're a marketer or founder
You'll rarely install an SDK yourself. What you need to know is whether the tool you're buying has an API, because that's what lets it connect with your other software and the services you already use. Ask the vendor what the API lets you do, then hand the detail to whoever builds your integrations. These are the questions I'd put to any vendor before signing up:
Want AI doing the heavy lifting in your marketing?
I build the systems that handle the boring 80 percent, so you get your week back. Done properly, with the human kept in.
- Is there an API, and what can it read and change in your account?
- Is there an SDK, and for which platforms and languages?
- Where's the documentation, and does it include code samples?
- Who on your side would need to set it up and look after it?
If a vendor mentions an SDK, it usually means they've made life easier for developers on a specific platform, such as a mobile app on Android or iOS. That's helpful news for your developer rather than a separate product for you to think about.
Which one should a junior developer reach for first
Read the API documentation first, because it tells you what the service can do at all. Then, if there's an SDK for your language, install it and use it, since it bundles the libraries, code samples and documentation that save you writing the same plumbing yourself. AWS's Boto3 for Python is a typical case of a vendor handing you exactly that.
If there's no SDK for your language, you can still call the API directly. My explainer on API vs webhook covers another part of how software talks to software, the best maps APIs list shows what APIs look like in a real category, and my page on the OpenAI Agents SDK is a working example of an SDK built around one vendor's services.
When you do use an SDK, keep the API documentation open beside it. The SDK shows you how to make a call from your language; the API documentation tells you what that call does to the service on the other end, and that's the part that matters when something breaks.
Do you make APIs, SDKs or developer tools? Tool makers can ask to be considered for a featured place on this list.
Want your tool on this list? Featured spots and reviews here are paid partnerships. Here's how to partner with me.
Related: python (this site).
Frequently asked questions
Is an SDK the same as an API?
No. AWS defines an API as a way for two software components to communicate, and an SDK as a set of platform-specific building tools. An SDK may include APIs.
What's inside an SDK?
AWS says an SDK often bundles libraries, compilers, debuggers, code samples and documentation.
Do I need to install anything to use an API?
Not the API itself; your code calls it. AWS says you install an SDK on your computer before developing an application.
Can an SDK be used to build an API?
Yes. AWS says you can use an SDK to create new APIs, which is one reason the two are layers rather than rivals.
Why do businesses use APIs?
AWS says APIs let you integrate predeveloped functionality in your own code and connect your software with preexisting modules and third-party services, so you don't build everything yourself.
What's an example of an SDK?
AWS's SDK for Python, Boto3, used to integrate Python applications with AWS services. SDKs also exist for Android and iOS development and for cloud platforms.
