Terrain and difficulty
There are two ratings for each cache: difficulty and terrain.
Difficulty
A number of people use the Markwell's Difficulty Guidelines v1.1, which are:
1. A ten year old could probably figure it out without too much difficulty 2. An average adult would be able to find this in about 30 minutes of hunting 3. An experienced Geocacher will find this challenging, and it will take up a good portion of an afternoon. 4. An extreme challenge for the experienced Geocacher. May require in-depth preparation or cartography/navigational skills. 5. Mensa or equivalent
Terrain
A number of people use the Scout Scale:
1. Handicapped accessible 2. Suitable for small children; generally on trail 3. Off trail; requires some risk of getting scratched, wet, or winded 4. Off trail; likelihood of getting scratched, wet and winded. Probably requires special equipment (boat, 4WD, etc.) 5. Requires specialized equipment and knowledge/experience (rock climbing, SCUBA, etc.)
Of course, these are just rough guidelines, and with 1/2 stars, you can rate your cache as being a little from column A, a little from column B.