wiseGEEK is a web site that provides free and clear answers to common questions.
I asked, "What is Decision Theory?" and got this concise answer.
Some other questions and answers in which you might be interested:
What is Thin-Slicing?
What is Foreshadowing?
How Should I Choose a PIN?
What Should I Consider when Buying a Gift for Valentine's Day?
Where Does the Practice of Wishing on a Shooting Star Come From?
wiseGEEK home page