Updated to include information from Storify CEO Burt Herman
To put it simply, Custom Timelines are kind of like a cross between Twitter’s List system and a curating tool like Storify. The intention for Custom Timelines is to enable users to select tweets from a feed to craft a cohesive story about a particular topic. But unlike Lists, which operate like a custom grouping of users, Custom Timelines allow for hand-picked specific tweets to keep in line with a specific theme. For example, Twitter #music curates the best celebrity tweets by sifting through individual entries by superstars. It can also be used for more subtle (and hard to automate) parameters, like Vines only.
Custom Timelines are, at this point, only available via Tweetdeck. They’re also geared towards hand-curating only — automated Custom Timeline building requires a custom API that would need a more experienced programmer to deploy with any elegance.
The feature is a great concept, if somewhat late to the party. At this point, a Custom Timeline would be best used as an embeddable tool to bring a curated Twitter discussion to another website — something that Storify has managed to do very thoughtfully for years. But, the convenience of being able to stay within Twitter (or, more specifically, Tweetdeck) to accomplish the same task isn’t worth sneezing at.
However, it would be more usable if it was available via Twitter’s website and had a much more user-friendly automated system. At this point, the Twitter stream can run so fast that catching all the details on a particular issue requires power-use tactics of only the best users. Also, it would be a great way to jerry-rig a repository for Vines, which often get lost in Twitter’s system. But it doesn’t feel like Custom Timelines is covering any new ground, and that’s not a great sign for the long term.
Update: Storify CEO Burt Herman sent a short statement to All Things D, which has a strong emphasis on collaboration between the company and Twitter: “We are exploring how we can incorporate the Custom Timelines into Storify, which lets you collect media from all social networks and anywhere online, along with adding your own text to provide context for stories.”