What are some things you should know when buying a digital camera?

One of the most important things you'll want to ask yourself when purchasing a digital camera is "What am I going to do with the 'final product'?" If you're going to use it primarily for regular printing, web page/computer graphics etc. Or will you be blowing up your images to 200x+ their original size?
Next, is it important that the camera is small and 'pocketable' or are there no size constraints?
Do features such as video capture, special digital effects and zoom. These will add cost to the camera, but increase its use possibilities.
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Be sure not to buy a "gray market" camera. Get your camera from a reputable source that honors warranties, etc.

Know what you want before you buy.

mainly we have to see how many mega pixels it is supporting.the more the pixels the more the clarity will be.the second thing is optical zoom.we have to see the extra features also .

SLR Point of VIEW:
Canon and Nikon own about 75% of the market (for good reason), Safety in numbers. Also mean you will have a grate selection in lenses.Once you start with a model of camera, you will probably spend more on lenses. The next time you need to replace the camera, you will not want to throw that investment.

Point & Shoot:
Only consider the 'optical zoom', 3x common, 10x is outstanding
Small digital cameras do great up to 6mp, after that you don't gain much. The Fuji 6mp F20 takes better pics than it 10mp cousin.
You should be able to shoot video at 30fps, with sound.
Should have at least a 2.5" LCD screen

Make sure you get it from a reputable firm like Amazon, Costco, or BHphotovideo.