Why is AES resistant to known-plaintext attacks? At least it's my understanding that AES isn't affected by known-plaintext. Is it immune to such an attack, or just resistant? Does this vary for chosen-plaintext?
A known-plaintext attack (i.e. knowing a pair of corresponding plaintext and ciphertext) always allows a brute-force attack on a cipher:


Simply try all keys, decrypt the ciphertext and see if it matches the plaintext.


This always works for every cipher, and will give you the matching key. (For very short plaintext-ciphertext pairs, you might get multiple matching keys. Then you need to try more pairs to eliminate the wrong ones).

If you have no known plaintext, only the ciphertext, you can do it similarly, but you also need a function which says whether what you decrypted is a plausible plaintext.

The problem with try all keys is that for every modern cipher (i.e. key sizes of 128 bit or more) the key space is that large that you need much more time than the remaining lifetime of the universe to check a significant portion of all keys.

So, the question is, are there any attacks which are faster than brute-force?

For now, there seem to be some attacks which are slightly faster (like needing only $2^{125}$ steps instead of $2^{127}$ for brute-force, a bit better for the 256-bit-key version) and needing either a really large amount of chosen plain- or ciphertexts (and knowing the result), or even larger amounts of known plaintexts. These are still not practically doable in our world.

There are no proofs that AES (or any block cipher) is really secure, only the heuristic "many smart people tried to break it and until now, nobody was successful" (or at least, nobody who succeeded told the public).