What is it?
A "carb" is a small hole in the pipe, usually located next to the bowl.
Examples of Carbs
How do you use a carb?
So basically the carb hole divides the smoking process into two steps:
1) You create the smoke by burning the dry herb (finger on carb).
2) You inhale the smoke (finger off carb).
Do I NEED a carb?
Short answer: NOPE.
Pipes with carbs are more popular; however, it ultimately depends on the smoker's preference.
Some prefer to smoke out of pipes with carbs to control the amount of smoke inhaled. Others don't necessarily feel the difference- especially when smoking small amounts of herb.
Plenty of pipes don't have carbs. (E.g. One Hitters).