"Complete Uppers" are usually just that...everything for the "upper" part of the gun is finished, headspaced, and usually fired. Includes the barrel, gas tube, bolt, charging handle, upper receiver, etc...
"Complete Lowers" are just that, too. Complete. The options you'll generally see floating around are:
1. Stripped Lower
2. Complete Lower (has lower parts kit installed), minus stock
3. Complete Lower HALF...everything's done, including the stock.
If you buy a Complete Upper half and Complete Lower half, slap the two together and you're good to go, in most circumstances.
I ordered a stripped Stag lower receiver from
www.ballisticsounds.com, and then a Stag rifle kit from
www.ar15sales.com, and the total cost was $728.
Buying separate parts saves on taxes, too!! If you buy an assembled rifle, cha-ching!