What is the difference between an API and an SDK?

An application programming interface (API) provides the rules and endpoints for software to communicate, whereas a software development kit (SDK) is a comprehensive set of tools, libraries, and documentation used to build applications. An SDK often includes one or more APIs within its package to simplify integration for developers.

Are all APIs web-based?

No, not all APIs operate over the web. While web APIs use internet protocols to exchange data between remote servers, operating systems and programming languages also use local APIs to allow software applications to interact with hardware components, file systems, or underlying software libraries on a single device.

An API is secure when designed with proper authentication, authorization, and encryption controls. System administrators restrict API access using API keys, security tokens, and strict permissions to ensure that only authorized applications can read data or trigger actions, while transport layer security encrypts data in transit.

Web Development

API

An application programming interface (API) is a defined set of rules that allows one software program to request data or services from another program.

also called: APIs, application programming interface

// definition

An application programming interface (API) is a contract and communication protocol that defines how software components interact. In web development, an API consists of specific endpoint URLs, request methods, and data formats, such as JavaScript Object Notation (JSON), that govern data exchange between systems. Developers write code that sends structured requests to these endpoints, and the server returns a predictable response containing the requested information or confirming an action.

Unlike a user interface, which connects a human user to a software system through graphical elements, an API connects two software applications directly without human intervention. While a user interface translates clicks and typing into system commands, an API uses programmatically defined code calls to perform backend operations.

// why it matters

APIs allow businesses to integrate third-party functionality into their websites and applications without building complex tools from scratch. By connecting to external services for payment processing, mapping, or user authentication, organizations reduce development time and maintain focus on their core product offerings. Internal APIs enable modular software architectures, allowing distinct engineering teams to update individual features independently without breaking the broader application. This architectural separation minimizes technical debt, streamlines system maintenance, and allows data to flow securely between separate databases and client interfaces.

// example

An e-commerce website uses an API to calculate shipping costs during checkout. When a customer enters their delivery address, the website sends an automated request containing the address details and package weight to a courier service's API. The courier's server processes the request, calculates the precise shipping fee, and returns the rate in a formatted response. The e-commerce platform instantly displays this cost to the customer before the purchase is finalized.

Questions and Answers

What is the difference between an API and an SDK?: An application programming interface (API) provides the rules and endpoints for software to communicate, whereas a software development kit (SDK) is a comprehensive set of tools, libraries, and documentation used to build applications. An SDK often includes one or more APIs within its package to simplify integration for developers.
Are all APIs web-based?: No, not all APIs operate over the web. While web APIs use internet protocols to exchange data between remote servers, operating systems and programming languages also use local APIs to allow software applications to interact with hardware components, file systems, or underlying software libraries on a single device.
Is an API secure?: An API is secure when designed with proper authentication, authorization, and encryption controls. System administrators restrict API access using API keys, security tokens, and strict permissions to ensure that only authorized applications can read data or trigger actions, while transport layer security encrypts data in transit.