Rolled out in Beta in July Twitter tailored audiences are now available for all.
Basically you can reach users on Twitter who have shown an interest in your brand or category; similar to Google RLSA’s.
Twitters example: “A hotel brand wants to advertise a promotion on Twitter and they’d prefer to show their ad to travel enthusiasts who have recently visited their website. To get the special offer to those people who are also on Twitter, the hotel brand may share with us browser-related information (browser cookie ID) through an ads partner. We can then match that information to Twitter accounts in order to show the matched users a Promoted Tweet with the travel deal. The end result is a highly relevant and useful message for the user. Advertisers will continue to receive the same reports that include how many users saw or clicked on an ad, without identifying who saw it or clicked on it.”
For more info visit the Twitter advertising blog