The basic construction of the banjo is fundamentally different than that of a guitar. The body of the banjo is essentially a drum head - a circular frame with material (plastic or animal skin) pulled taut over it. The neck of the banjo is bolted on to the circular frame. The strings run from the base of the frame up the neck, and hold the bridge in place. Because of its construction, the banjo has a thin, tinny sound - which isn't to say it is unappealing..
There are quite a few types of banjos - including banjos with 4, 5 and 6 strings. The 4-string banjo is generally tuned to CGBD, and is often played with a pick.
The 5-string banjo includes a "short" partial fifth string, which despite sitting below the lowest bass string of the banjo, is actually tuned higher than the other strings. The 5-string banjo is tuned many different ways, but one common tuning is G(partial string)DGBD. This type of banjo is generally played via the use of a thumbpick and two fingerpicks, or alternately bare fingers.
The 6-string banjo is essentially a guitar - it is usually tuned to the standard guitar tuning of EADGBE, and uses a guitar neck. The 6-string banjo is a favorite of many guitarists, as it doesn't require learning a whole new set of chords, thus the transition from guitar to banjo is relatively easy.
To learn more about the history of the banjo, read this concise profile of the banjo on About.com's World Music site.