A hashtag is a type of label (or metadata) used on social network and microblogging services which makes it easier for users to find messages or posts with a specific theme or content.

Users create and use hashtags by placing the hash character "#" in front of a word or phrase. This word or phrase must not be interrupted by spaces or other non-numeric or non-alphabetic characters).

The hashtag is either placed in the main text of a message or at the end of it.

Searching for a hashtag on a social network or micro-blogging service will present each message that has been tagged with it.

Twitter has a widget to show tweets that have been tagged with a certain hashtag. The widget can be embedded in a blog or website as explained here: How to embed a timeline (widget) on your own website or blog?.

In Wikipedia : Hashtag