A system exists to further a set of goals, usually put forth by the system's designers. A hacker hijacks the same system for a different set of goals, one that may be contrary to the original ones.

This is obvious in the hacks of ATMs, casino games, consumer reward plans, and long-distance calling plans. The goal of whoever managed the ATM is to dispense cash to account holders and deduct the appropriate money from their accounts. The goal of the hacker is to receive cash without having money deducted from their account (or without even having to have an account at all). Similarly, the goal of a casino is to be fair (which means equal opportunity between players, not equal opportunity between players and the house). The goal of the hacker is to tilt that advantage in their favor.