tuning can be a broad statement.... consider that most of the internals in your honda-acura engine are pretty darn good off the assembly line. but they don't use the really really good parts. that's why there are better, lighter, more efficient and tougher replacements for pretty much everything.
a 'built engine' is one that has the goods and has had it's computer changed or reflashed or chipped or something, (i'm no expert).... read tuner mags. if you can't afford them a lot of magazines have a .com with archives. this post is getting way too long.