Custom Made Mountain Bikes
A custom-built bike is one in which the frame itself is built exactly to you measurements.
Basically, when one talks about a customized bike - or rather a custom-built bike - one is talking about the frame. You send along your measurements to the manufacturer - length of foot to knee, knee to pelvis, torso, overall height and they construct the frame with those measurements in mind. They then put on all the rest of the stuff - the high-end tires, the brakes and shifters you require, the crank set, and you're good to go.
All that costs money. :o
An Easy Customization - the Paint Job
There's other ways to customize mountain bikes, of course, though I haven't seen many people do it. Why not give them a stylized paint job like you see on lowriders? Of course mountain bikes used for actually going down mountains (as opposed to those used for cross-country or even road riding) take a bit of a beating and can get scratched up, but for most bikers, a hand-painted mural on their bike can be quite attractive. Why let the lowriding crowd have all the fun? The only reason why I haven't painted my own bike in orange and black tiger stripes is because I know I don't have the skill to pull it off - but if you've got any artistic talent at all, why not go for it? 😉
all the best
magic