As Sandi mentioned, SLR stands for single lens reflex. You are looking "through" the lens at the viewfinder to see what the lens sees. This has some advantages and disadvantages. The P&S cameras have the display which is a lot eaiser to hold above crowds, take pictures of yourself and a friend or just get some interesting shot angles. But you don't always get the same picture and exposure you saw in the display.
The main advantage for me is the lack of shutter lag (no more fish butts

). A DSLR or film SLR will meter, focus and take the picture in milliseconds (the camera I have now will take five frames per second, Great of action shots). The buffer is larger so you can take them faster and then the camera will write them to the memory card.
A P&S camera can have a huge lag and can miss a lot of action shots because of it. Another plus is the interchangeble lenses. A P&S camera has the lens on it. They do offer some optical zoom (the more, the better) but with a SLR camera you can get different lenses for different options. Long distance close ups, macro, wide angle, etc. Plus, usually the better lenses are more sharp and offer better exposure options for you as well.
hope that helps.......