The Proterozoic Eon was a time in Earth's history from about 2.5 billion years ago to about 542 million years ago. It followed the Archean Eon and preceded the Phanerozoic Eon. During the Proterozoic Eon the planet's crust and geophysical processes settled into patterns similar to what we experience today. For example, plate technonics worked much as it does now. However, since the world was younger and hotter, the plates scooted around on the earth's crust surface with greater speed. Life branched out into new forms such as plants, fungi, and animals during the Proterozoic Eon. The "Precambrian" is a term that can refer to all the Eons preceding the Phanerozoic, although in actual practice it is most often a reference to the Proterozoic, and specially the time of interesting biological innovations in the later era of the Proterozoic (the Neoproterozoic).

1. The page about the Proterozoic Eon at the University of California in Berkeley, which also has a good page about life of the Proterozoic.
2. A discussion of the geology of the Proterozoic with an emphasis on Missouri and North America.
3. A good introduction to the structure of geologic time and the eons with a brief description of the Proterozoic at the Idaho Museum of Natural History.
4. The rather witty Paleos page about the Proterozoic Eon.