These are the primary ways an attacker can get into a system:
Physical Intrusion - If an attacker has physical access to a machine (i.e. they can use the keyboard or take apart the system), they will be able to get in. Techniques range from special privileges the console has, to the ability to physically take apart the system and remove the disk drive (and read/write it on another machine).
System Intrusion - This type of hacking assumes the attacker already has a low-privilege user account on the system. If the system doesn't have the latest security patches, there is a good chance the attacker will be able to use a known exploit in order to gain additional administrative privileges.
Remote Intrusion - This type of hacking involves an attacker who attempts to penetrate a system remotely across the network. The attacker begins with no special privileges. There are several forms of this type of hacking. Note that Network Intrusion Detection Systems are primarily concerned with Remote Intrusion.