Per the WC sticky-
Tubing and Fittings
Tubing can be as generic as the hardware store vinyl tubing at your local Ace Hardware or as complex as the Feser, Primochill, Masterkleer or Tygon. All of these brands are very flexible and come in most commonly used ID and OD tubing sizes for watercooling. If you are needing some good bends made, consider getting good tubing, but understand most is upwards of $1.50-$2.50 per linear foot.
Fittings are one that cause a lot of confusion for most newcomers. Here is a simple way to understand the terminology that accompanys each:
I.D. - Inside diameter, most commonly referring to the ID of the tubing to be used. 1/2"ID means that wall to wall, the inside of the tubing measures 1/2" (metric is also used and is measured in millimeters or mm)
O.D.- Outside diameter, similar to the ID, the OD simply is the measurement of the tubing through the cross-section from one side of the outer wall to the other.
G1/4" - This is the one that confuses the most people. This refers to the threaded fitting standard that is used by almost all waterblocks and radiators. It is the end of the fitting that gets screwed into the block or rad; the other end of this same fitting is measured with the I.D. standard for tubing size.