Algorithms are (as you know) a series of specific steps towards achieving a goal that will always achieve that goal. KBSs and similar AI-related things use heuristics, which are like rules of thumb. They work most of the time, but not all the time. They are useful when an algorithm is impractical. For instance:

Goal: prevent hijackers from boarding a plane.

Algorithm:
- search the plane
- strip search all passengers and crew as they board
- search all luggage before putting it on the plane

Heuristic:
- question and possibly search any passengers that match a predefined hijacker profile
- put passengers through a metal detector
- X-Ray all luggage.

The algorithm is totally impractical, but is guaranteed to work. The heuristic is practical, but is not guaranteed to work, although it will work almost all the time.

This is closer to what we use ourselves to make decisions.