The Phanerozoic Eon is the time from about 540-542 million years ago until now. It begins with the Paleozoic Era, which in turn begins with the Cambrian period. The key defining feature of the Phanerozoic Eon has been the activity of multicellular animal and plant life (although multicellular organisms certainly existed in the late Proterozoic Eon). In fact, "phanerozoic" means "revealed life" or "visible life" (Greek phanero-, phaner-, -phane, -phanic, -phanous, and -phany all have this meaning of showing, making visible or manifest or open). Originally proposed as the name for our eon because this was the eon when life became manifested, it now turns out that life (at least single-celled life) was around for about three billion years before the Phanerozoic Eon even began. Everything that precedes the Phanerozoic is sometimes refered to as the "Precambrian" time. As multicellular life forms, we humans are most interested in the Phanerozoic Eon, and would be most interested in similar Phanerozoic Eons on other planets.

1. An introduction to the periods of the Phanerozoic provided by Chris at his enjoyable peripatus site.
2. This page at the University of California at Berkeley explains how we invented the geologic time scale.
3. Frederick M. Soster's very fine introduction to geologic time.
4. Don McBride offers a historical timeline covering the geological history of the planet. Interestingly, Don started the site using Hypercard, a program the authors of the historylines.net website also used in the 1980s and early 1990s before html came along.
5. As usual, the Paleos page about the Phanerozoic Eon is quite enjoyable.