Microtags are short personal hashtags.
Example: "Deployed my first Hello World ASP.NET MVC application on mono #dev"
Example: "Eating ice cream with the #kids"
You (or your software) will be able to ignore #dev tweets if you are not interested in my development work. You may ignore #kids tweets if you do not want to know when we are having ice cream in the local zoo.
Microtags tell your followers what a tweet is about. Your microtags are sub-channel names inside your twitter stream. Microtags can be used by twitter clients to show only relevant information. You can choose tags as you like.
You should tell your followers which microtags you have, so that they can configure their client. One way to tell your followers is to register your microtags here (registration currently in internal beta). If you publish your microtags, then your followers (or their software) know what your tags mean. If you invent new microtags, then your software may publish them and clients can fetch the meaning of new tags automatically.
There will be more.
Until then see the sample microtags list: http://microtags.org/tags/?http://twitter.com/wolfspelz for the twitter account @wolfspelz in YAML.