While most of the information listed by
Billows is accurate, there are a few corrections/additions worth noting.
One must achieve a total of 12 points for the three Higher Level tests, thus averaging a score of 4 on each. High scores and low scores can balance out; however, if more than one elementary score is received, the candidate does not receive the diploma. (Elementary is defined as a 2 or below on a Standard Level, and a 3 or below on a Higher Level test; elementary scores can also be received on the extended essay or on the Theory of Knowledge essay.)
The 150 hours of "CAS" incorporate creativity (not community), action, and service.
As a graduate of the program and a diploma recipient myself, I can say definitively that it is a very painfully rigorous standard for students in the United States; rumor has it that academic standards are higher in other countries in the world, but from my experience, American high school students don't have to work very hard, and IB is rather a shock to some. Fortunately, universities in this country are beginning to recognize IB scores on a much larger scale; University of Tulsa, for example, grants 30 credit hours and considerable scholarship money to an IB diploma recipient. Other universities waive their core curriculum requirements and/or grant certain sums of credit hours.
The joys of procrastination go hand in hand with the work of IB. Sites such as ibscrewed.net allow a forum for overworked, stressed, sleep-deprived students to gather and let off a little steam.