Glides, These are probably one of he most important parts of your cabinets. Think about how many times you will open and close a drawer. Glides should be durable and smooth.

Shown here are full extension ball bearing glides.
Lets break that down:

Full Extension means just that, the glides extend fully so that you have free access to all the draw. If you have a draw that mostly opens, but you still have the back part under the cabinet, these are not full extension. If you are buying quality custom cabinets, FULL EXTENSION SHOULD BE STANDARD.

Ball Bearing. This means that the glides slide in and out on little metal balls instead of metal or wood. This give a smooth felling to the drawer.

Soft Close - You still have to push the drawer in, but at the last inch or so the glide grabs and pulls the draw close so there is no bang. (Soft close glides can be install on the side of the box (side mounted or under the box- under mounted. I find that the undermount soft close glides are more reliable then the side mounted.)

There are numberous manufacturers of quality hardware.
BLUM is very well know and pricey.
There is also HAFELE less know, high quality and sometime less expensive - I think the are equal if not better in some hardware then Blum.
Knogt & Vogt (KV) They also product quality hardware
(Note: Boxes stores like home Depot and Lowes bmay have Blum glides, but they are not necessarily the higher end glide.)

Stay away from the old white glides that move on themselves. If you are paying for custom cabinets, you should get ball bearing glides.