"A hashtag is a word or a phrase prefixed with the symbol
#. It is a form of metadata tag. Short messages on microblogging and social networking services such as Twitter orInstagram may be tagged by putting "#" before important words, either as they appear in a sentence, (eg. "New artists announced for #SXSW 2012 Music Festival!") or appended to it.
Hashtags provide a means of grouping such messages, since one can search for the hashtag and get the set of messages that contain it."
(Source: Wikipedia )