A Difficulty/Terrain Challenge is a type of Challenge Cache.
A Difficulty/Terrain challenge is where you have to find a given number of caches with a specific difficulty and/or terrain rating. While a Difficulty/Terrain Challenge seems similar to a Fizzy Challenge, they are different in that they focus on specific D/T combinations and commonly require multiple finds for those ratings. An example would be to find 20 geocaches with a terrain of 3 and a difficulty of 5.
Specific rules vary depending on the goals set by the challenge owner. The rule most commonly found amongst these challenges is that Liars Caches are not allowed.