Web patents - what can be patented?

Hi, The stories about web patents seem so bizarre. Like Yahoo claiming to own patent on showing ads based on some metrics. I wonder what all can and can not be patented. As in, how come Friendster or MySpace could not get online social network patented or how come Amazon could not get online book store patented? Basically what I want to understand is - what are the features of a product for it to be patentable? Thanks. Manas