What is an application programming interface (API)?

API stands for Application Programming Interface and refers to a programming interface that enables the Communication between different applications enables. External programmes can gain access to certain components of a software via an API and transfer data.

Unlike with a binary interface, the programme connection takes place at source code level takes place. Operations are carried out via standard commands so that compatibility with different programming languages is guaranteed. Among other things, an API can be based on Databaseshard disks, graphics cards and user interfaces.

The advantage of a programming interface is the Easy integration of new application components into an existing system. In addition, APIs are usually documented in detail with their associated parameters.

How does an API work?

Programming interfaces (API) are used in particular by developers to allow their programmes to dock with another. A programming interface specifies how data can be received and sent. The commands and data types that an API accepts are defined in protocols. They are used by the corresponding components for uniform communication.

A basic distinction is made between Internal/private APIs and external/open APIs. Private programming interfaces can only be used by programmers within an organisation. This optimises work on internal company processes. In addition, they are protected from unauthorised access by certain security measures. External APIs are available to the public in directories for integration into other systems. However, sometimes the use of an API is restricted or subject to a fee.

Application areas for programming interfaces

APIs can be used to connect a wide range of processes:

Weather forecast

Global weather data from a wide range of international sources are retrieved via programming interfaces and can be displayed to the user via app on the smartphone.

Appointment booking

Service providers can use APIs to enable their customers to make bookings on online portals or search for specific services. These can be, for example, appointment information at doctors' surgeries or the comparison of flight prices. The website connects to the programming interfaces of the respective service providers and generates an overview with the most suitable options.


Retailers use APIs to control the inventory of their products and provide customers with information about availability.

What is the difference between API and REST API?

REST is an abbreviation for Representational State Transfer and refers to a software architecture, which is guided by the principles and behaviour of the World Wide Web. A REST API is a specific form of an APIused for data transfer on distributed systems. Compared to a general API, the REST architecture has the following features six design principleswhich must be adhered to by developers:

Uniform programming interface

The resources are accessible via a specific Uniform Resource Identifier (URI). Different operations can be performed using HTTP methods via the same URI. Suitable formats for resources are, for example, JSON, XML or text.

Independence of client and server

Client and server applications must be decoupled from each other. The client should need nothing more than the URI of the respective resource.


To increase the scalability of the server and improve the performance of the client, resources can be stored in the cache.


Rest APIs do not require information about sessions. If the server requires data about the client session, this is sent via a separate request.

Multi-layer system architecture

Between the client and the server, there may be a number of other applications that communicate with each other. The client cannot see through how many servers the response was transmitted.

Code on demand (optional)

In most cases, static resources are transferred via REST APIs. Sometimes, however, it can also be executable code such as Java applets. This should only be executed on demand.