Speedfreak, no problem. I had asked the exact same question at one time.
This is a cut and paste from Cathar's Watercooling FAQ.
"The tubing carries the water from component to component. Tubing can come in a variety of materials and sizes. It is of course best to choose the size of the tubing to match your components fittings.
When considering the size of the tubing and matching it to your components, it's important to understand some of the terminology of fitting and tubing sizes. There are two measurements, being the "Inner Diameter" (or ID) and the "Outer Diameter" (or OD). The ID of a piece of tubing is pretty much all that you should be concerned with. The OD of tubing is near meaningless for our needs since it just defines how thick the walls of the tubing is when you know what the ID is.
For fittings, it's important to not confuse ID and OD. You want the "Outer Diameter" of your fittings/barbs to match the "Inner Diameter" of your tubing. It's usually okay if the tubing's ID is up to 1.5mm (or 1/16") too small for the OD of a fitting, as most tubing can typically be stretched at least that much to fit. If you cannot match up a hose with a fitting size (because you may have fittings of differing OD sizes), then you will need to use a hose sizing adapter."
