Whats the difference between a bass guitar and a regular guitar?

There are a few significant differences between a bass guitar and a regular guitar.  A bass guitar typically has 4 strings (though it can have more), is larger in body & neck, and has higher string gauges (string thickness).  A bass guitar is used for keeping the band on track by keeping the rhythm and hitting the root notes of the chords.  

A regular guitar typically has 6 strings, and can be used for a wider variety of purposes.  These include keeping the rhythm with chords, playing foundational riffs, or improvisational solos for instrumental layering.  The regular guitarists are almost always the more well-known of the two.