Projectors 101

You'll need to understand a few things when looking at projectors:
Aspect Ratio
Throw
Resolution and
Brightness (or lumens)

The Aspect Ratio is a number like 4:3 or 16:9 - but the easiest way to think about it is TV versus movie screen. All the old tube TV's were 4:3 (because of the limitations of a Cathode Ray Tube, but that's another discussion). Movies were presented in the widescreen format of 16:9. I would recommend getting a projector that supports both formats, but then pick one and stick with it. Fiddling with the aspect ratio can lead to some weird presentations. I personally like the 16:9, especially for movie clips.

Throw can be either short or long. This is the distance between the projector and the screen. Basically if you are going to have your projector close to the screen you want a short throw and if you are going to have your projector far away you want a long throw.

The resolution of your projector is closely tied to the aspect ratio. Basically the aspect ratio is the reduced fraction of the resolution. So a resolution of 800x600 is using the 4x3 aspect ratio. A resolution of 800x450 is using the 16:9 ratio. If you're wanting to use a lot of video clips, then you'll probably want a higher resolution projector. The problem is when you start throwing around letters and number that aren't related. There's VGA, XGA, UXGA, WUXGA, HD, 1080p, 720p and a slew of others. What you can do to same some sanity is to compare apples to apples - they should all have a number-by-number resolution (e.g. 1024x768). Compare those numbers (check out the whole story here).

Finally, brightness is measured in units called lumens. Basically the more lumens your projector can put out, the brighter it will be and the easier to see with ambient light in the room. You might think that you would want to max out on the lumens, but the higher lumen rigs are more expensive, heavier, and cost more to maintain. So there are some trade-offs to consider. For most portable applications you will want something in the 2000 lumen range - that should make it bright enough to handle significant ambient light, but still keep it small enough to move without a team of oxen.

Now it's time to shop. I usually start out looking at a bunch of reviews and see what matches up with my needs and my budget. Happy hunting.