Hello Old Doll,
A bridge is usually just one part of a song that has some melodic or lyrical contrast to the rest of the piece (verses & chorus). Often, the bridge will contain some different chords than the rest of the song.
Usually the bridge has different melody than the other parts. Most songwriters use a bridge to further dramatize the lyric somehow or to somehow offer a different perspective on the song's topic.
Not all songs have a bridge. Most pop songs have some sort of bridge usually after a couple of verses and maybe a chorus or two. I guess it is called a bridge because it is near the middle and connects the beginning and ending of the song?
A chorus, on the other hand, is a part that typically repeats after verses and usually carries the overall theme or idea of the song. Sometimes a bridge takes the place of a chorus in between verses.
Listen to just about any popular song and you'll be able to spot the bridge - it's the bit that sticks out from the rest.
"That darn Pythagorean Comma thing keeps messing me up!"