On the web, the term router can refer to different concepts depending on the context:
For the network layer, a router is a networking device that decides where to direct data packets.
For a single-page application (SPA) in the application layer, a router is a library that decides what web page is presented by a given URL. This middleware module is used for all URL functions, as these are given a path to a file that is rendered to open the next page.
The concept of routing in SPAs has evolved significantly over the years. See the hash routing glossary entry to learn more.
In the implementation of an API in a service layer, a router is a software component that parses a request and directs or routes the request to various handlers within a program. The router code usually accepts a response from the handler and facilitates its return to the requester.