Middleware often sits between the operating system and applications on different servers and simplifies the development of applications that leverage services from other applications. This allows programmers to create business applications without having to custom craft integrations for each new application.

Typically, middleware programs provide messaging services so that different applications can communicate using messaging frameworks like Simple Object Access Protocol (SOAP), Web services, Representational State Transfer (REST) and JavaScript Object Notation (JSON). The systematic tying together of disparate applications, often through the use of middleware, is known as enterprise application integration (EAI).

At a basic level, middleware provides services required to connect applications together such as concurrency, transaction management, threading and messaging. More sophisticated implementations of middleware principles are baked into modern integration infrastructure such as enterprise service bus (ESB) and API management software to provide greater governance, risk management and accountability.