John the Ripper is a very popular program made to decipher passwords, because of the simplicity of its playability and the multiple potential incorporated in its working.
John the Ripper doesn't need installation, it is only necessary to download the exe. in our computer and start using it without any kind of problem, accessing the file where we have saved it with the command line. It might be that in some occasions our anti virus warns us about some danger, of which we don't have to worry about and simply delete this warning, since its use is totally safe to our computer.
John the Ripper passwords decipher functioning is based on brute-force attack, which consists in deciphering the key by individual tries of all possible combinations until finding the perfect one. For that, John the Ripper also counts with the dictionary attack, since many passwords are created with words taken from the dictionary, because for the users it is much more simpler, since remembering large passwords that include rare letters, capital letters, numbers, etc is quite hard and boring, even being safer.
The fact that John the Ripper is so popular deciphering passwords is because it allows checking easily if the passwords are strong enough to protect the account, and also the great variety to customize it, defining the algorithm that should be used as a model to try all possible passwords, including certain rules about the letter's variability, offering the possibility of starting over, etc.
It is not necessary to say that the use of this kind of software should always be done in a responsible way, taking advantage of all the possibility offered to ethical and security control ends offered by our passwords.