80% means they're incomplete, there's still work left to be done on them such as drilling a few holes, threading the buffer tube hole, and applying the finish. These are not recognized by the BATF as a complete firearm (as the BATF is interested in the AR-15 lower receiver, where the serial number is stamped), so you don't have to do an FFL transfer to buy them. You'll probably need a set of drill bits to drill most of the holes, and a fixture set to make sure you get the holes in the right place. Check out cncgunsmithing.com for more information and some photos of the fixtures, bits, 80% lowers, etc. You can generally get an 80% lower for, well, about 80% the cost of a fully mfg'ed receiver. It's unlikeley your quality will match that of Colt/RRA/Bushy/DPMS/insert favorite manufacturer, but you'll have a real intimacy with your rifle because of your part in its creation.