Intro:
Normaly the USERNAMES and PASSWORDS are saved in the database in encrypted form. If u are sucessfully able to get the the USERNAME and PASSWORD of a website or a server. It is very rare that you encounter an unencrypted USERNAME and PASSWORD.
Normaly USERNAMES and PASSWORDS are encrypted using MD5 algoritm. So when you get USERNAME and PASSWORD you will encounter a 32 digit hash. The problem then is to decrypt that hash to get admin access. This TuT explains and teaches how to decrypt that hash.
An MD5 hash can not be decrypted using reverse engineering or by using the reciprocal of the MD5 algorithm. In the system when a password is entered then the system apply MD5 algorithm on the given password and then compare it with the already encrypted password saved in the database.

The two methods to crack an MD5 hash are:
(1) Dictionary Attack
(2) Brute Force Attack

I am going to explain both attacks.
Dictionary Attack:
Basically in a dictionary attack the cracking software use all the dictionary words(use the readymade wordlist) to crack the hash. 
Internaly the program first take a dictionary word then apply MD5 algorithm on it then compare it with the given hash(u want to decrypt). If same then you get an unencrypted password otherwise it goes to the next dictionary word. If the password is a dictionary word then this attack will be successful otherwise use brute force. Dicrionary attack is comparitvely faster than brute force attack.
Brute Force Attack:
In a brute force attack a set of chracters(lower and upper case alphabets, Arabic numorials i.e 0-9 and special chracters) are given to the software. The then softwares tries all possible combination(Digits can be limited) of the chracters to guess the password. 

Internaly the program first take a combination then apply MD5 algorithm on it then compare it with the given hash(u want to decrypt). If same then you get an unencrypted password otherwise it goes to the next combination. Keeps on doing untill you get an unencrypted password.
Example:
If your password is a dicrionary word like apple,porn etc then it should be cracked using a dictionary attack. But if your password is like this 4Dmin0,Fu(k0 etc then brute force will be successful.
Softwares:
I personally recommend "CAIN"
Download link of CAIN: