Hacking is the act conducted toward breaking or any attempt to break into computer systems. Cracking is a bit different from hacking but in some occasion hacker uses cracking techniques to accomplished job. Cracker focus on breaking authentication security on software and systems while hacker can write, use or modify program to attack any level of computer and/or network.
There are 3 types of hackers; they are classified depending on regular tendency and intention. Some hackers are not harmful but most non-professional hackers are harmful and fraudulent because they hack for financial gains, like hacking individual, commercial and government websites and systems for payment, stealing of credit cards, hacking bank account and transferring of money from various bank accounts to another. As hackers are well known for attacking systems so as crackers are known for bypassing software and application authentication, and those who target and attack on or through telecommunication systems are called Phreakers.
Either hacker, cracker or phreaker they are called hacker because a hacker may equipped with cracking and phreaking skills, a cracker will remain cracker while a phreaker will remain phreaker but both can develop gradually learn to become hacker because hacking required programming skills.