Reblogging (or, in Twitter parlance, reposting and formerly retweeting) is the mechanism in blogging which allows users to repost the content of another user's post with an indication that the source of the post is another user.
It was first developed by Jonah Peretti at Eyebeam Art and Technology Center's R&D program under the project 'Reblog' (from where the term originates) as an open-source tool for individually-run blogs. Tumblr then built it into their social network for re-sharing posts within the network, and similar features ("Retweet" on Twitter, "Share" on Facebook) then followed.
For a number of microblogging and social networking services, reblogging has become a means of both social bookmarking and user commentary; unlike social news services like Digg, Slashdot, and Reddit, however, reblogging typically does not involve a centralized "front page" to which the highest-ranked post is appended.
Reblogging (and the increased attention paid to the indexing and encouragement of reblogging) has become a major feature of many social networking sites and content-hosting services, and it has also become a potent means of secondary content promotion and audience measurement whereby links to external content are syndicated across multiple profiles and the reposts are indexed as a measurement of currency and relevance.