Promoted tweets help you reach to your targeted customers. Promoted Tweets appears at the top of the search results page.
A Promoted Tweet isn’t guaranteed to stay afloat for a long time — if the tweet isn’t tracking well in terms of replies, clicks, and a number of other metrics Twitter is calling “resonance”, it will be pulled, and the advertiser won’t pay for it.
Watch a nice video to understand the promoted Tweets: http://vimeo.com/10910517
Read an article to know more about the promoted Tweets: http://techcrunch.com/2010/04/12/full-details-on-twitters-long-awaited-ad-platform/