While ikiwiki supports hierarchically categorising pages by creating
[[SubPages|ikiwiki/SubPage]], that's often not flexible enough, and it can also be
useful to tag pages in various non-hierarchical ways.
Since this is a wiki, tagging is just a form of linking. The general rule
is that all tags are links, but not all links are tags. So a tag is a
special link that ikiwiki knows is intended to be used as a tag.
Generally you will tag a page without putting a visible link on it.
The [[tag_plugin|plugins/tag]] allows you to do so, like this:
\[[!tag mytag othertag thirdtag]]
You can also tag a page with a visible link:
\[[!taglink mytag]]
This tag will be displayed just like a regular [[ikiwiki/WikiLink]].
One way to use these tags is to create a [[ikiwiki/blog]] of pages that have a
particular set of tags. Or just look at the [[BackLinks]] to a tag page to
see all the pages that are tagged with it. [[Plugins]] can be written to do
anything else with tags that you might desire.
Once you have tags, you can use the [[plugins/pagestats]] plugin to
generate tag clouds.