The MD5 hashing algorithm is a one-way cryptographic function that accepts a message of any length as input and returns as output a fixed-length (32 digit hexadecimal number) digest value to be used for authenticating the original message,also called a message digest.
MD5 is one in a series of message digest algorithms designed by Professor Ronald Rivest of MIT (Rivest, 1992). When analytic work indicated that MD5's predecessor MD4 was likely to be insecure, Rivest designed MD5 in 1991 as a secure replacement.
The MD5 message digest hashing algorithm processes data in 512-bit blocks, broken down into 16 words composed of 32 bits each. The output from MD5 is a 128-bit message digest value.
MD5 checksums are built to be non-reversible, meaning that you can't look at the checksum and identify the original inputted data. With that being said, there are plenty of MD5 "decrypters" that are advertised as being able to decrypt an MD5 value, but what's really happening is that they create the checksum for lots of values and then let you look up your checksum in their database to see if they have a match that can show you the original data.