A hashtag is a word or a phrase prefixed with the #. Example:

#Wikipedia is an #encyclopedia that anyone can edit.

Hashtags provide a means of grouping such messages, since one can search for the hashtag and get the set of messages that contain it.

When clicking on a hashtag, a feed of related posts by people and pages about the topic can be read. Uppercase and lowercase does not matter for hashtag.

A hashtag is meant to define a popular topic on post.