Adding onto limecool’s response, both iOS and Android are able to use them. I’m using wefwef right now on my iPhone. It looks like any other app on my phone and acts a lot like it, too. (As a former Apollo user, I can only commend the wefwef team for a truly spectacular replication of Apollo’s sleek user interface. The similarities are truly striking.)
So they’re kind of like a glorified web bookmarks, but they have some capability for managing their own storage (note when you’re prompted to “update” wefwef) rather than being simple links. As an iPhone user, another notable difference comes when you’re getting a new phone. These days, all your apps redownload whenever you restore from a backup, which of course takes time. But your webapps? They’re ready to go right away.