Trails is a little tricky to explain. The site, in alpha version for now, was one of the web apps we saw being demoed at SXSWi's Startup Spotlight. It looks like Pinterest, but functions very differently. Essentially, Trails lets you curate bookmarks which are filed under topics or used as answers to questions others have asked, and these "trails" take the form of pins spread out on your web browser.
Each Trail initially has a topic or a question that acts as the main item "pinned" to the browser spread. "How do I learn to make a website?", for instance, or "Easy Eats in Downtown Austin." When you click into a Trail, you'll see all the bookmarks by a curator, collected across different places on the web—they can be sites, videos, images, lists, etc. These either serve as the answers to the initial question asked, or act as a collection of items catalogued under the cover topic. Curators also have the ability create a "path" using these bookmarks, defining the particular order their audience will see the web items they've collected.
Trails has high aspirations, including eventually building a database of respected curators who will act as authorities on certain topics, and the ability for regular people—with continued use of the app—to increase their influence on others. But do we really need another bookmarking website, even if the premise is slightly different from what we've seen before? Only time will tell if Trails will catch on to a crowd of adopters and make it out into the mainstream.
If you're still confused and would like to try Trails yourself, you can check out the Alpha site the team is using to demo the app at the conference: http://trails.by/sxsw.