Custom timelines are an entirely new type of timeline — one that you create. You name it, and choose the Tweets you want to add to it, either by hand or programmatically using the API (more on that below). This means that when the conversation around an event or topic takes off on Twitter, you have the opportunity to create a timeline that surfaces what you believe to be the most noteworthy, relevant Tweets.
Looks almost like a shot across the bow of Storify. If you’re using it to curate Tweets and nothing but, you can now do it natively within Twitter – and do so without replying on a third party. That said, Storify has much more power than that, as a genuine multi-platform curational tool.
Neville has created a good example of it in use.