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There are different kinds of projectors, but keeping it simple means that they fall into two categories - DLP and LCD. The DLP projectors use a color wheel to create the image while the LCD projectors use LCD panels. Both give you a nice image but there are differences that you can see (such as the level of contrast and how "black" the blacks are). Viewing the projector you're interested in is like test driving a car from the dealer's lot- you need to do it to see if it meets your preferences.
Another consideration is the resolution of the projector. If you want the highest resolution possible today at home, then it needs to be 1080p (which matches what a Blu-ray disc can do). Going with 720p might be fine, especially if the high-definition you're watching is mostly coming from HD channels off the TV, and will cost significantly less.

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Also whether how powerful the zoom lens is (powered or manual) can be a consideration as well. Plus whether or not the lens is a "short throw" - not needing as much room between it and the screen or wall in order to properly project its image.
Of course both types of projectors use bulbs and the stronger the bulb, the brighter the image will be.