OMG, NOOB ALLERT! On Twitter, hashtag is some sort of link algorith in which the server can track what is written afterwards. So, for instance, if I want Benitez sacked, I type #rafaout. If there are a lot of people worldwide doing the same hashtag, it becomes a trending topic (because it is being highly discussed/used)... On Instagram, the hashtag is used to help on the search engine and to place your photo into certain categories. For instance, if I take a pic of SB, I can type #cfc, #chelsea, #stamfordbridge, #london and #epl. Whenever someone searches for any of the words I hashtagged, my photo will appear. However, stupid people are using it everywhere. Specially on facebook statys and photos, even if they have no utility...