The bedrock of all great tunes, the bass guitar gives life to the rhythm of a song. A strong bass line can turn a mediocre song in to a number one. If you see anyone dancing to music, they're dancing to the bass line.
Traditionally a 4-stringed instrument, many modern electric basses incorporate a 5-th and sometimes even a 6-th string to provide a greater scope of playing, reaching lower notes and higher notes than the standard 4-tring. It's not been unheard of for a bass player to have up to 10 strings.
The electric bass has a longer neck than a standard electric guitar, so if picking up the instrument young, it's generally best to start on a 3/4 size bass. The bass is also a great instrument for learning the fundamentals of music, as most bass parts in modern pop music are single note melodies, making it easy to play along with while learning notes and scales.