One mistake many people make is thinking that MD5 is something they can use for encryption. This is a common misunderstanding. MD5 is not encryption like you may think, but a cryptographic hashing algorithm. While MD5 is a form of one way encryption, it's not encryption in the standard sense.
So what is encryption, and what is a hash? Encryption is the transformation of information with an algorithm. Generally, encryption requires the use of a key to tranform the information. Encryption makes the information unreadable unless it is decrypted. Decryption is the process of using the key to make the information readable again.
A hash is the result of running a cryptographic hash function against amount of data. This hash is created by running a mathematical function on the data. In the case of MD5, the hash returned is a sequence 32 hexadecimal charecters.
So MD5 is not capable of encryption your data in the way one may assume. MD5 can be used for many purposes. Often, MD5 is used to verify the integrity of a file or string. You include a MD5 hash of a file when distributing it to verify the integrity of the file.