Web server vs Application server vs Database server
Web server serves a web content (HTML and static content) over the HTTP protocol.
Application server is a container on which one can build and expose business logic and processes to client applications through various protocols like HTTP. Some cases it will have internal web server. Application server is heavy in terms of resource usage.
Database server refers to the back-end system of a database application using client/server architecture. The database server performs tasks such as data analysis, storage, data manipulation, archiving, and other non-user specific tasks