There are various words that can be used to refer to trying to get the plaintext from a ciphertext, including decipher, decrypt, crack, and cryptanalysis.
Often the process of trying to break cryptography is referred to as an "attack".
The term "hack" is also sometimes used, but it has other connotations, and is only used informally.
People who try to decrypt messages are called cryptanalysts; more informal terms like hackers and crackers are sometimes used, generally with the implication that they have bad intentions.
Being a cryptanalyst is generally a good thing to do though: people who use encryption systems actually want to know if they have weaknesses, and don't want to wait until the bad guys find out for them.
It's like a security guard checking doors on a building; the guard hopes that they can't get in, but if a door is found unlocked, they can do something about it to make sure the bad guys can't get in.
Of course, if a security guard finds an open door, and takes advantage of that to steal something for themselves, they're no longer doing their job properly!