What are the 'parts' to a mechanical disc brake set? So there are rotors, pads, brake lever, brake cable...I just want to make sure that if i buy brakes I can get all the parts and I'm not missing anything.
It depends on how you buy the brake, so you need to check whats coming with what you're buying. Typically, you get the caliper+pads+rotor+mounting hardware and buy the brake lever (which is a V-brake lever most of the time unless its a road brake in which case its a regular short pull lever) separately along with the cables+housing. In some cases though (esp. buying brakes on eBay), people buy just the caliper+pads+mounting hardware and get rotors separately.

This example from Nashbar tells you the Avid BB7 Road comes with "Includes 1 post mount (74mm) caliper, 1 rotor (160mm) with bolts, sintered pads, and 74mm post mount to IS mount adapter".

The typical hardware (Avid BB7 Road with a G2 rotor) is shown below:


(from Nashbar)

To this you'd need to add a brake lever (e.g. Tektro RL340), some brake cable and brake housing in order to get a functioning brake system. 

For maintenance and some description of the important parts, see this Park tool repair guide as well.