IMBA have a very nice page on grades of trail difficulty. It's handy to be familiar with this when trying to describe trails. A summary table is this:

Full page here:

http://www.imba.com/resources/trail_building/itn_17_4_trail_difficulty.html

On our site we talk about types of trail. Here's a quick idea of what we mean by the these:

Single Track - Enclosed

Single track is pretty obvious - a trail only wide enough to ride single file.

The enclosed type is bordered at both sides by vegetation or rocks or fences or something. This type of trail is a rider's dream!

Both varieties of single track are also referred to as 'sniggle' (a term that allegedly originates from the MTB-OZ mailing list and a common typo for the work single).

Single Track - Open

Still too narrow to ride anything but single file, but out in the open (with nothing to crash into normally).

Double Track

Wide enough to ride side by side (and often to drive on), but not a full on fire trail or 4WD road (ie. not graded or with a centre line of grass/rocks/etc.).

Firetrail

Trail wide enough to drive a vehicle on with no centre divide. To MTB riders there's no real difference between a firetrail (built by the fire services for access and protection) and other 4WD tracks or forest roads so we classify them all under the same thing here.