Mountain biking has three main types:

• Cross Country (XC).  As the name suggests, this is over a variety of terrain, with climbs and descents.  Courses can be A to B, out-and-back or in a loop.  Bikes are lightweight, usually with front (and sometimes rear) suspension.
• Downhill (DH).   No prizes for guessing which direction this is!  Courses can be steep and over rough terrain with jumps and drop-offs.  Needs a chunky bike with big knobbly tyres and serious suspension.
• Four-cross (4X).  This like a BMX-style race but downhill and on mountain bikes.  Should-to-shoulder with lots of jumps and turns it's exhilerating but not for the fainthearted.

A list of trail centres can be found at: http://www.ukmountainbiker.com/view/Information/Trail_Centres/

There are lots of other local routes to explore: http://www.ukmountainbiker.com/routes/East_Midlands/

There are also your local cycling clubs which do organised Mountain Bike rides.  Visit the club finder section of the website to find out more about how to get involved.