Twitter became a publicly traded company this month and the stock price nearly doubled out of the gate. The service is considered so valuable because of the huge numbers of customers that use it and the ability it has to display ads to those customers.
Twitter's "Promoted Tweets" options explains that ads can be targeted to users based on gender, geography, keywords in the user's tweets and even the user's device of choice. Additionally, an advertiser can submit a list of emails that Twitter will compare to the database of Twitter accounts.
The criteria used for tailored ads may seem obvious since they are based on Twitter tracking the use of their own service, but Twitter also tracks users elsewhere on the web.
Twitter calls it "tailored suggestions" and analyzes a user's web browsing to determine which other tweeters to suggest users follow.
"Tailored suggestions" works because many sites around the web have embedded one or more Twitter functions, like a button or widget.
Individual tweeters can turn off "Tailored suggestions" or "tailored ads" by going to the bottom of their "Security and privacy" options in Twitter's settings.