The Verge has a great piece on the new feature:
“Stories: a rolling compilation of snaps from the last 24 hours that your friends can see. You create your Story as you go about your day by tapping “My Story” above the friends you want to send a snap to. Or, you can tap a new shortcut button in the app’s camera screen to instantly post a snap to your Story. But unlike conventional snaps, Stories don’t disappear in a puff of ephemeral smoke after you’ve watched them. You can watch a friend’s (or your own) Story over and over.”
Each Story is the sum of all the snaps you’ve added over the last 24 hours, which means its size is always fluctuating. What doesn’t change is that every piece of the Story is less than a day old, so viewing one might be the fastest way to see what a friend’s been up to. Snapchat may not look much like Facebook, but with Stories, the company is taking its first steps toward competing with Facebook’s most important product: News Feed.
Rather than falling for pressure from users to add a “send all” button or allow for groups, the company appears to be pushing for the new Stories feature as a way to share moments with all of your friends.