This instrument shares its name with one of your fave *NSYNCers, Lance Bass, but it's pronounced 'base'. Drums add the beat to your song, but the bass is the real backbone of a song. The bass line of a song is a pattern or a couple of patterns that are repeated throughout your tune.
The bass is the instrument that puts the 'funk' in your tunes and is the main instrument used in funk music. It looks almost identical to a guitar except the bass only has four strings, instead of six like a guitar. There are special bass guitars that are available with five and even six strings, but they aren't very common. Another feature that's different from a guitar is that the strings on a bass are thicker. Most bass' don't have frets either, (the sections separated with bars on a guitar) which help you figure out where to place your fingers on the neck.
The way you play it is different too. You don't pluck it like you would a guitar, you 'slap' it using your thumb or you can use your other fingers to pick the strings. Some people use a pick, but your fingers will toughen up with calluses after a while. The bass produces a low, deep sound. One of the most well known bass players is Flea from the Red Hot Chili Peppers. He's got a mean slap and his standout bass lines set the Chilis sound apart from a lot of other bands. Another talented bass player is Sting. We're talking real talent here, as he not only plays bass but sings lead. Few bass players sing lead because the bass line and the lead melody are different. Think of it as the musical version of patting your head and rubbing your stomach at the same time. What are you waiting for? Pick up a bass and start slappin'!