a lower reciever (lower piece) look like this:

it has the magazine well, and the places for the trigger, hammer, sear springs, stock, mag release and all those fun little things about 40 parts in all... i dunno ive built two of them... one STAG one CMMG. all parts are the same but a piece of advice if you built your own lower get the lower parts kit from the same manufacter as the reciever. You can usually buy a reciever for about 125-160 and a parts kit for about 50-65 dollars. Those will build you lower reciever without a stock. the stock, tube, buffer and spring will cost you about another 45-80 dollars depending on kind of stock. a complete lower will look like this.


or you can just buy a complete lower from an FFL for about 270-280

if your building a lower for the first time please realize you need to be patient and it helps to print out a parts assembly diagram from online. Bushmaster has a pretty detailed one on there website;
http://bushmaster.com/anatomy_bushmaster.asp

but look around...
if you have any other questions just ask :-)