Great question, Froggy.
They are in a progressive hierarchy in this order:
1. Knowledge is the ability to acquire information and retain it.
2. Intelligence is the ability to apply knowledge regardless if it is understood.
3. Understanding is to perceive and comprehend the nature and significance of knowledge. It is to perceive the hows and whys of knowledge.
4. Wisdom trumps them all. Intelligence can apply knowledge for good or evil, but wisdom is not just the capacity of doing something; wisdom is knowing whether or not something should be done and then congruently doing it or congruently not doing it. We can get along without knowledge, intelligence or understanding, but life sucks without wisdom. Furthermore, a moron with wisdom is smarter than an erudite without it.