One of the big differences between different digital cameras is the size of the sensor. We a continuing megapixel race between all of the manufacturers. The interesting point to think about is "Are all all megapixle measurements the same?" The answer is not what you would think.
The short answer is that a megapixel measurment is consistent and so a camera that has 15 megapixels will have the same resulting image size regardless of the camera. The issue really has to do with pixel density. That is imagine the sensor is the size of a coin. 15 megapixels on a dime will require much smaller pixels than the same image size on a quarter. This is the big difference between dSLRs and P&S cameras. Pretty much all dSLRs have larger sensors then the average point and shoot. This translates into better image quality since the required pixel size to achieve the resolution is larger. It also means that the dSLRs handle low light situations better since larger sensors mean greater light sensitivity.
If you are interested in view in this graphically, I suggest that you check out this link. It graphically shows the difference sized sensors used in various cameras. It definitely puts this entire conversation in perspective.