There are four-string, five-string, and even six-string basses. If you are starting out, go for a four-string. You can play pretty much anything on a four-string bass, and it's best not to complicate matters when you are just getting started. The advantage of the five-string is that it goes lower, down to a B. Modern funk and pop often use these lower notes for a heavy bottom. The six-string bass extends the instrument's scale on both ends.