Orange County Community College explains that cyclins are one of many proteins within a eukaryotic cell that regulate the cellular life cycle. All cyclins belong to the same genetic family, which contains several different subtypes.
The Medical Biochemistry Page explains that eukaryotic cells require cellular-cycle control mechanisms, such as cyclins and other control proteins, to progress from one phase of the life cycle to the next. Cyclins are heavily involved in cyclin-dependent kinases, which act as switches to trigger the next phase in the cell's life cycle. The website also explains that these kinases can only be activated by cyclins, making them a crucial component for the survival of every eukaryotic cell. OCCC explains eukaryotic cells have become too complex to simply shift from one phase to another on their own, so they evolved the use of these cyclins over time.
There are some subtypes of cyclins that are far more complex and execute different roles in the life of the eukaryotic cell, including the retinoblastoma protein known for its bond with the E2F protein. This type of cyclin is heavily associated with various forms of cancer, proving just how crucial healthy cyclins are to the body.