In short.....the "upper" includes the top portion of the receiver where the ejection port is located. The lower includes the bottom portion (magwell, trigger assembly, etc.). Sometimes you'll hear "complete" upper which can include the barrel assembly, handguard, front and rear sights and so on. That is also true of the lower...stripped lower, complete lower (includes pistol grip, sometimes includes the stock). I'm an AR "noob" also. There are others on this board that know alot more, and might have a better explaination for you. You should check AR15.com , lots of good info.
Like everything on the net.. just sadly more so at ARFCOM ... take your time and filter through the BS. There is plenty of it on ARFCOM. If you find something that just doesn't make sense.. OK even if you think it makes sense.. post it over here and someone will be able to vette' the data or call BS.