Multicellular life kick starts! So many animals evolve in such a short space of time, that their appearance in the fossil record is sometimes called the Cambrian explosion.
While the Cambrian explosion certainly is an important event; the most important thing about the Cambrian is actually the advent of animals with mineralised body parts. Soft body parts don't tend to preserve well; while shells and exoskeletons will readily fossilise in lot of different conditions. Our knowledge of the deep past depends very much on what has been preserved; so the Cambrian really marks the beginning of a readily available fossil record.