Difference between hashing and encryption

Neither MD5 nor SHA-1 is an "encryption" algorithm. Both are "hashing" algorithms.

In the simple sense, something that is encrypted can later be decrypted to get the original back.

Something that is properly hashed stays that way; it is a 'fingerprint' of the original pattern that cannot be 'dehashed' to reconstruct the original.

Ideally, there is only a single unique hash value that can be derived from each unique string of characters.

Of course, it's easy to prove that that's not possible when hash values are small -- small as in 160 or 256 or 512 bits. But in practice, because the strings of characters aren't random, because they're patterns of words more than just patterns of letters, the number of likely hash values gets smaller.

The value of unique hashes is that they can be used to verify that the original string hasn't been changed. If I send you a message along with a hash, you can run the message through the hash algorithm to see if the same hash value comes out. If it does, you can feel confident that you're reading exactly what was written.