Well they name them after real life things. So just think of what other real life thigns with those names do... in a similar way they work for computers too.

Bridge for example. It simply creates a connection between 2 points. Real life talking about 2 roads or when talking about an RG-6 cable that needs a TSplice, or a dsl modem. It's just connecting. It's not doing anything to it other than connecting and allowing traffice or signal to move from one spot to another.

A switch and be thought of as "or". A light switch is either on OR off. Same with a 24 port switch with computers connected to it. Port3 is talking to Port6 OR not.

Router is a little more complicated. But it really just makes routes or defines a path for info. Just like a person that's planning on what roads go where, he is a router. He's telling things where they can and cannot go.