For your price issue, some are more expensive then others because they are built with better parts, leading them to run a bit better then the cheaper counterpart. Now for at the models.
Macbook: The basic, midrange laptop offerings. Now as powerful as the Macbook Pro, but can outrun the Macbook Air.
Macbook Air: A thin, very portable laptop. The most expensive, not as powerful, but it is a quite nice machine.
Macbook Pro: The most powerful of the Mac laptop range. Also the most expensive.
iMac: The middle range Mac desktop computer. Quite a minimulistic design, and isn't the most powerful in the price to power ratio when compared to other computers, but if you want a Mac desktop computer this may be for you.
Mac Mini: A small, lower end Mac. The case you get that is the computer is very small, and if you bought it you would also need to buy a screen, keyboard and mouse.
Mac Pro: The high end, more business of heavy graphics work Mac. Quite expensive, though very powerful. Also, like the Mac Mini, you need to buy a screen, keyboard and mouse.