In the most fundimental sense, the "geologic column" is a time line.
It is divided into the preCambrian and the Phanerazoic, which are in turn subdivided into smaller units, which are subdivided into even smaller units, which are (often) subdivided into even smaller units.
For the Phanerazoic (Paleozoic, Mesozoic, Cenozoic), the divisions are (as I understand it) defined on the presence or absence of certain fossils (ie the type of life present in those time periods).
Most prominently, the ends of the Paleozoic and Mesozoic are marked by major extinction events.
Moose