A stub is an article which is clearly too short, but not so short as to be useless. In general, it must be long enough to at least define the article's title, which generally means 3 to 10 short sentences. Note that a longer article may be a stub if the topic is complex enough; conversely, a short article on a topic which has a very narrow scope may not be a stub.
Another way to define a stub is an article so incomplete that an editor who knows little or nothing about the topic could improve its content after a superficial internet search or a few minutes in a reference library. An article that can be improved by only a rather knowledgeable editor, or after significant research, may not be a stub.
If you make or find a stub, add this special text to ask other users to make the article longer. The easy way to do this is to use the short-cut: [[Include(Stub)]] See also Stub/Talk.
For a list of stub pages you can improve on RocWiki, see Stub Pages List.