Oh the world of hash. Many people get hashing and encrypting mixed up. My employer, though I've told him the difference, still refers to the practice of hashing as encrypting. A hash is basically a one-way transformation of an item to a fixed length string. MD5 and AES are 2 popular ones. It's pretty much impossible to guess the original content of a hashed item, and it's also pretty much impossible to find 2 items that have the same hash.
Well, that's what we'd like to think, but it's not entirely true. "The Story of Alice and her Boss" is a short read on MD5 collisions. It's nice they actually gave 2 files that have the same md5 sum (a25f7f0b29ee0b3968c860738533a4b9 done with MD5summer)