Hacking is the act of gaining unauthorised access to a computer. A hacker will break in by circumventing security, they may know someones password, or they may have found weaknesses in software that allow them to access the computer.
Once a hacker has access to a computer they will be able to access the data on it as if they were an authorised user.
Hacking may take place remotely or even directly on the computer.
The most common form of hacking is someone simply logging onto a computer with someone elses username and password. This is why it is important to have a strong password that no one else knows.