I will try to clarify a bit for you
Paralax is a movement of the cross hairs on the target when you move your eye behind the scope. To see it, lay the scope, or the rifle with the scope on it on a solid surface and look through it without touching the rifle. If you move your eye the cross hairs will "wander" around the target.
"AO" means that you have the ability to focus your scope at different ranges, and when it is focused properly there will be no paralax movement at that range.
The first number is a measure of magnification, a 4x scope will magnify your target four times normal size. The variables, most commonly a 3x by 9x, has an adjustment ring and you can select what power you want for the situation.
The last number, 28, or 42, or 50, is the size in millimeters of the "objective lens" at the front of the scope.
The reticle (cross hair) is most commonly a "duplex", meaning that the center portion you put on the target is a very fine line, and away from the center it is commonly thicker, and easier to see in bad light.
"Mil Dot" is a term that refers to a set of dots on the cross hair that will assist you in judging range, and in "holding over" for longer range, or holding off to the side to compensate for wind.
It is very true that you get what you pay for, more money will get you a better scope. You did not indicate what amount you want to budget for this item. You do not know yet if you will like long range shooting, and with that said I think you should look for a quality scope, with a good guarantee in the lower price ranges. I just purchased one of these, and I have not had it to the range yet, but I am pleased with the clarity, and the looks of it. And it has a lifetime warantee from Bushnell, even though it does have the name tasco on it.
http://www.eabco.com/Tasco02.htm
My budget will not presently allow me to drive a Ferrari or shoot a Leupold, your priorities my vary from mine.
Good luck with your choice.