In microblogging, a tag is a short label that can be included in the content of your post. It should describe the subject of your post in such a way that someone searching for posts about that subject would be able to find relevant posts. For example, if I post about something that's happening in Leeds, I include the tag "#leeds" somewhere in the post. If someone wants to find out what's going on in Leeds, they can then search for all the posts that have this tag.
Tags provide a flexible, organic system of labelling which is unlike classificatory taxonomies in that the labels do not have to be fixed, and new labels can be created as easily as new content. As such they are well suited to decentralised and distributed knowledge systems, but less useful for canonical reference works where a hierarchy of nested classes can provide structure to data that would otherwise be unnavigable.