Cyclins were originally named because their concentration varies in a cyclical fashion during the cell cycle. (Note that the cyclins are now classified according to their conserved cyclin box structure, and not all these cyclins alter in level through the cell cycle. The oscillations of the cyclins, namely fluctuations in cyclin gene expression and destruction by proteolysis, induce oscillations in Cdk activity to drive the cell cycle. A cyclin forms a complex with Cdk, which begins to activate the Cdk, but the complete activation requires phosphorylation, as well. Complex formation results in activation of the Cdk active site. Cyclins themselves have no enzymatic activity but have binding sites for some substrates and target the Cdks to specific subcellular locations.
The function of cyclin is to regulate the progression of the cell via the cell cycle. They are grouped into four classes basing on their behavior. You can get some more information on http://en.wikipedia.org/wiki/Cyclin#Function.