Tagging

Tagging conventions change with the application, so the best guide you have is what's already been done on that particular site. That being said, here are some additional guidelines: 

Tagging works best when the tags are as simple as they can possibly be without being too vague. Start with the simplest language and then increase detail; shorter words generally make better tags. When choosing a tag, imagine yourself as a user who's only access to the content is that single tag. What do you expect to see when you click that tag?

For example: You want to find a picture of a yorkshire terrier. What tag would you click? The first one would probably be "yorkshire+terrier", if it's there. If not, you look for the next closest tag that applies with decreasing levels of detail. Maybe "toy+dogs" or "lap+dogs", followed by "terrier" and finally "dog" would be your last resort. All of these tags should be included.  

As noted by gtoure, there really aren't any rules. It depends on the application, and on you. 

Why are  you tagging? You could be tagging for yourself, so that you can find your stuff later. Or you could be tagging for social reasons - because you want other people to find your stuff. Or for applicative reasons - for example, GeoTagging.

Depending on your reasons for tagging, and on the application you're tagging in, you may want to choose tags that enhance "findability" by other people, as described by gtoure, or choose tags that make sense to you, and perhaps only to you. 

For example, on delicious, there are 203,912 links tagged with "toread". This tag is obviously useless for anyone else but the person who tagged this link.  I've also seen people who uses "secret" tags whose meaning is clear only to them.

On Yedda, people tag their questions with topics mainly for findability by others. Tagging a question with topics that enhance its findability increases the chances that the relevant people will find the question and answer it. These tags are also used by Yedda to automatically find these people and invite them to answer your question.  

If you're interested in further reading on this, check out the following:

A cognitive analysis of tagging by Rashmi Sinha

Why do People Tag? by Joshua Porter