Can I just copy-past the text from < somewhere >?
No, you can't. While there can be some useful info in the begining of wikipedia articles, or articles on other websites, you should not just copy-paste it. It is always much better to rewrite that information with your own words. Take a look at this question for additional info.
Well in that case, what should I include into the tag wiki?
Everything which will be useful for someone who will open the wiki to learn what is the tag about. I would suggest the following:
The excerpt should contain a very brief description of the tag's subject. If the tag is about an anime/manga series, a good idea is to tell what is the series, and what is it about. Here is a decent example:
Cowboy Bebop is a 1998 Japanese anime series directed by Shinichirō
Watanabe. It follows the adventures, misadventures and tragedies of a
bounty hunter ("cowboy") crew traveling on the Bebop, their starship.
If the tag is about some general stuff, the excerpt should tell people exactly what is the tag about. Here is a decent example (from culture):
Questions about anime and manga in the broader context of Japanese culture. Includes both questions about cultural origins for anime/manga references and questions about the cultural impact of a particular series.
However, don't make the excerpts too vague. After reading the excerpt, one should be able to understand if the tag suits well for his/her question.
- The info should contain more detailed information about the tags. This may include a slightly more detailed plot explanation, related media (anime/manga/games/etc), links to official and fan-made resources, etc. Basically, the tag info should provide enough information to briefly get acquainted with the subject. Don't make it too long, though, we are not going to re-write wikipedia articles.
How about links? Should I include links?
- you shouldn't leave the links without explanation. A short (1-2 sentences) explanation of where the link leads to is more than welcomed.
- obviously, don't leave links to anything illegal :P
- try to always add a link to the source if you are quoting any material.
- don't leave too many links. Usually links to 2-3 extensive sources about the subject are enough.
Anything else I should know?
Well, just a couple more things that may seem obvious at the first glance, but are important:
- Don't add useless text. "Use tag XXX for questions about anime and manga XXX" is probably not a good tag wiki text.
- No spoilers. Try to avoid being too concrete while describing the plot. Remember that people that will open the wiki may just be starting to watch/read manga/anime, so don't even spoil anything that happens in the 2nd chapter of a 500-chapter manga.
- Add any info about the tag usage that you think is necessary. For example, if there are two tags, say, tag XXX and tag YYY, which are similar to each other, it is a good idea to explain where to use XXX, and where to use YYY in their tag wikis.
What do I do with existing tag wikis that I believe are low-quality?
Obviously, edit them to make them better.
Most SE users have faced a situation in which they were not sure how to tag a question properly, or whether a specific tag is suitable to their question. Well-written tag wikis help to deal with this problem, and reduce the amount of incorrectly-tagged questions on the site.