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.