When you have a string, it is relatively straightforward to convert it to a MD5 hash by using software or programming functions (or even manually by hand if you follow the steps of the algorithm!). However, say you have a MD5 hash: 912ec803b2ce49e4a541068d495ab570 . How do you convert it to a string? The simple answer is that there is no algorithmic way that this can be done. The MD5 algorithm is performed in such a way that it only hashes in one direction from a string to a hash of the string. You cannot reverse the steps in the MD5 algorithm and expect to get the original string from a hash.
However, even though MD5 hashes cannot be decrypted using an algorithm, they can be decrypted using Rainbow Tables. There is a good chance though that the hash may not be stored in a Rainbow Table due to the large number of input string possibilities.