Grouping and classifying things.
Often in hierarchical structures -- For example:

(So you have lotsa little things) Writeups

(Which are part of a bigger group) Node

(Which in turn are part of a bigger set) Web Site

(And so on to the next level) WWW

(And on to an even higher level) Internet

(And so on and on and on) Human Made Things

(Until you have a set that includes) Everything

See also: Taxa and Taxon

The most common biological taxonomy is cladistics; you might also be looking for biotaxis. The biological taxonomy you're most likely to be familiar with is based on The Five Kingdoms, although The Three Domains have since been added to the top.