They are typically referred to as "80% lowers". So long as you finish the machining yourself (even if that finish is pushing the start button), they are legal. Quality varies significantly though. They will even do build parties where a shop will setup their CNC for the day to just run the 80% lowers. However the ATF doesn't take kindly to them because they typically will have the same company doing the machining that sells the 80% lowers and it gets just a little too easy.
EDIT: Technically according to the law, any gun that you build yourself is legal for possession but not sale. The 80% complete state is where companies have found the ATF to not give them significant hassle on so they stay there. And yes, any lower that was built from an 80% state cannot legally be sold or given to anyone else, ever.