It would help if we knew the make / model of a router.
In lamens terms.. a routers job is to "route" internet traffic to individual devices. Phone, Pc, Laptop etc. Wirelessly and wired. You can have a "all in one" modem and router. Or you can have a modem (usually comes from the isp, comcast, etc) and router seperate.
Are you having certain issues with connectivity or...?