Well duh! Finally! Twitter wants your money. Twitter has started advertising in our timelines to begin making money from its user base. Twitter has started offering "promoted tweets" that are visible in a user's personal profile based on followers and interests.
In the past three years since Twitter's launch, this is the first monetizing strategy that has been implemented.
Twitter has 25 million global users and with these promoted tweets, advertisers can get access to them. Tweets will be clearly marked "promoted" so that they can be distinguished from other tweets. Twitter plans to tailor the advertisements to the interest levels of its users for more personalized marketing.
(P.S. FYI Twitter also charges Google and Bing over $10M a year to access its content and make it visible in search engines.)
Here's what a promoted tweet looks like: