Hash or Message Digest is a digital finger print of a plaintext block. All the information of the message is used to construct the Message Digest hash. The size of a Message Digest is always the same, independent of the size or content of the message from which it was created.
If two files or text has same hash then content of that will always be same. The message cannot be recovered from the hash. For this reason, Message Digests are also known as one way hash functions.
There are various hash functions but md5 and sha1 are most popular. This tool generate md5 and sha1 hash of any text string.