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

Alongside the breadcrumbs for this item which shows its "ancestry" in concept space, the Child items tab lists its descendants, and the Siblings items tab the concepts that exist under the item's "parent item".