Geological classification

History, at least from a geological standpoint, is very heavily categorized and classified.

At the top level (more or less-- right from the get-go it's a little confused), you have the eon.  There are four of them, going back essentially to the beginning of the Earth.  The first three are also grouped into one supereon (see, I told you it's confused) called the Precambrian, which isn't very interesting to most people since it covers the really early years when there wasn't much life on Earth and only three television channels.

The eons are divided in turn into eras, of which there are twelve in total. Or fourteen, depending on who you ask.  Some of the earliest ones were apparently borrowed from the moon, as if it had any to spare, since there isn't much evidence left around on the much more geologically active Earth.

The eras are divided into periods.  There are at least twenty of these-- much longer than a typical hockey game, although not nearly as long as a baseball game.  Those things go on forever.  You may have heard of the Jurassic Period, for example, notable for its remarkable fossilized animatronics.

The periods are divided into epochs, the epochs into ages, and then the ages (unofficially) into chrons.  After that point, I get the impression that geologists just give up and start cataloging events as happening "that Thursday, you know, after the thing, when all that stuff happened."

It's worth noting that all of these terms are used to refer to time periods in geology.  If you want to talk about the actual rocks, each one has an equivalent-- to wit, (super)eonothem, erathem, system, series, stage, and chronozone.