If you want to target more precise sets of users, you can create custom segments by specifying certain @usernames that are relevant to the product, event or initiative you are looking to promote. Custom segments let you reach users with similar interests to that @username’s followers; they do not let you specifically target the followers of that @username. If you’re promoting your indie band’s next tour, you can create a custom audience by adding @usernames of related bands, thus targeting users with the same taste in music.
Twitter Advertising Blog, Interest targeting: Broaden your reach, reach the right audience
What does that mean? Twitter will use your personal browsing history, people you follow and any other data it has about your activity in order for advertisers to microtarget their ads to you.