What are cyclins and cyclin-dependent kinases? what do these proteins do in a cell?

Cyclin-dependent kinases (CDKs) are enzymes that phosphorylate different substrates involved in the progression of the cell cycle. These phosphorylated substrates are usually activated and promotes progression of the cell cycle. Cyclins, on the other hand, are cofactors for the CDKs that activate them by binding to it. Derangements in the CDK-cyclin complexes will dysregulate the cell cycle.