Mountain biking is the sport of riding a bicycle offroad. A mountain bike is the bike used for mountain biking.

You get different types of mountain bikes:

• Fully rigid
• Hardtail
• Softail
• Full suspension

Here is a picture of a Fully Rigid:

Here is a Hardtail:

Here is an example of a Soft Tail:

Finally a Full Suspension:

For more information check out the Wikipedia article.

Riding a bike and doing things that a road bike with thin tires can't do in places it can't go.