Electric bass is what most people refer to as a bass guitar. Pedants such myself like to correct people misusing the term. A bass guitar is effectively a normal acoustic guitar, but bigger, with four strings usually tuned EADG an octave below a standard guitar, and sounding an octave below the written score. The electric bass is arguably derived from the double bass, despite its appearance, taking its place in the band lineup. Hint: don't get too worked up about the difference.
Five- and six-string basses are also available, usually tuned BEADG and BEADGC respectively. Eight-string basses are also available, although rare, and, like a twelve-string guitar, have four doubled-up strings tuned an octave apart.