Definition
An application server is a software environment that hosts an application’s business logic and serves application-level requests to clients. Typical examples include Tomcat and Jetty from the Java ecosystem, and web framework runtimes in other languages such as Node.js and Gunicorn.
How it works
Requests usually arrive at a web server or reverse proxy first, and dynamic requests are then forwarded to the application server, which runs the application code, queries the database and produces a response. It manages runtime resources such as process or thread pools and connection pools, letting developers focus on business logic.
Usage
Almost every website or API service that needs dynamic processing runs on an application server: e-commerce checkout, content back ends and model inference endpoints all live in such environments. In deployment, a single VPS typically runs one or more application server instances.
Related and confusable terms
An application server is not the same as a server: the former is a software concept (the program running the application), while the latter can mean either the hardware or the process listening for requests. It is also on a different layer from SSH — SSH is a protocol used to administer these servers remotely.