A guitar is a stringed instrument common to nearly all genres of music. There are several different varieties, but most have six strings and are either acoustic—the sound is amplified from from the body of the guitar; or electric—the sound is captured by a "pickup" then amplified through a speaker.
A bass guitar is a string instrument common to rock, reggae and jazz bands. It is usually part of the rhythm section. Bass guitars usually come in 4 or 5 string varieties.