Effects of attacks

Another way of classifying exploits is by their effect.

Takeover attacks
These let the user take the machine over, or at least cause it to do something unpredictable to the owner but predictable to the attacker.
Denial Of Service (DOS) attacks
These are just disruptions. An example of this is flood-pinging a machine to slow down its networking to the point that it's unusable. DOS attacks are notoriously difficult to deal with, and often must be handled in a reactive rather than proactive fashion.

As an example, there are very few systems that can't be brought to their knees by a malicious local user although, with such tools as the ksh's ulimit builtin command, you can often minimize these attacks.

Using these divisions, you can look at a system and see which classes of attacks it could potentially be vulnerable to, and take steps to prevent them.