My good deed for the day:

1: Tight or "track" geometry means the bike will have a ride that is very agile and almost "twitchy". Lax or relaxed geometry will be the opposite; it will offer a very smooth easygoing ride. A general rule of thumb for telling how relaxed the geometry of a frame is would be to check wheelbase measurement (longer usually = more relaxed) and head tube angle, Steeper head tube angles will offer a twitchier ride. (This isn't always true but it's the best shorthand description I could come up with.)

2: The common frame materials are steel (usually chromoly) and aluminum. (there are others like titanium, magnesium, and carbon fiber but we won't get into those here.) Al is ususally a little lighter weight but offers a 'harsher' ride (the frame doesn't soak up the bumps in the road as well). Generally within a certain price point steel will be a bit heavier but offers a cushier ride. Cyclists have been arguing about which is better since the first Al frames were built but at the end of the day it's a personal preference. For casual street riding I (and many others here) prefer steel.

3: There is no shame in running 2 brakes on a fixed gear but if you only run one brake it should be the front. The front brake provides over half of the braking power for the bike. You can slow or stop the rear wheel using back pressure on the pedals.

If you aren't already familiar with the ins and outs of fixed gear bikes and general bike mechanics your best bet is probably to get a factory made FG bike (bikesdirect, IRO, 1/8th inch, etc.) and take it to your LBS to make sure everything is assembled correctly before riding. Pedaling a fixed gear bike is one of the most fun things you can do with your clothes on but it can be dangerous if you do not have your bike set up correctly. Please take some time to read the stickies in this forum, the articles about FG written by the late Sheldon Brown, and if you still have questions come back and ask here.

Good luck and happy pedaling.