For bitcoin, it's the inverse of the target that the hash must be less than. Litecoin uses scrypt instead of SHA256, so I'm unsure what the difficulty indicates.
Litecoin's difficulty means mostly the same thing as it does for bitcoin. It's the ratio of the maximum target to the current target.
Difficulty within (but not between, for obvious reasons) each can be compared the same way. A difficulty of 100 means it takes, on average, 100x as many hashes to find a valid block as it does at a difficulty of 1.