Twitter will be pushing mobile advertisments through their official Android and iOS applications, much like LinkedIn began doing just a month ago. You won't start seeing pop-up ads like we do in something like Angry Birds for example, but rather "promoted tweets" will begin to appear in your main timeline.
At present, the promoted tweets and trends are only seen when you access the search pages within the app. Twitter have said though that initially only a small number of users will begin to see these tweets appear on their timeline from brands they follow.
Twitter have tried to pretty it up and make the whole deal sound more user orientated -- but let's face it, it's straight up advertising. Mobile ads are here to stay though, and with 55% of Twitter traffic coming from mobile, it was only a matter of time before we saw something like this emerge.