What does it mean when there's a # sign next to words?

[Mel]

I've seen a lot of this lately and have been wondering...
Ex:
#cuteandsexy
#weekendfun
#fbing

What's up with this # next to a bunch of words?

[Derp guy]

taging for exp. you see a pic of the beach then you see this #beach #sand #water #fun

also its the cool thing

it rly has no reason to be their

[Little_Tadase x3]

It's mainly used in Instagram. It means you're tagging something. Like if you posted a picture of you and your best friend, you could put #bestfriendsforever #sistas or something like that.

[Heath]

It's called a hashtag, for use with either twitter or instagram, they're basically just topics like "Going to a party this weekend! #weekendfun
A hashtag is a word or a phrase prefixed with the symbol #, a form of metadata tag. Short messages on microblogging social networking services such as Twitter, Facebook, Tout, identi.ca, Tumblr, Instagram, or Google+ may be tagged by including one or more with multiple words concatenated, e.g.:
#Wikipedia is an #encyclopedia

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

[Ryan]

The # is also called a hashtag. It is a new trend on social media for people to attach hash tags to their statuses. For example: Answering questions on yahoo answers #questions #yahooanswers. People can also view all Instagram photos that have a specific hashtag so it can be a way to get more likes.

[Daniel]

It's a hashtag, it's a twitter thing