Here is what Apple has to say:
With web apps, the power of the Internet meets the brilliance of multi-touch. And suddenly, iPhone and iPod touch can do that much more.
Flick through lists of news articles on Digg. Play games like Sudoku and Bejeweled with the touch of a finger. View movie times, train schedules, and blogs.
Web apps dont just extend the functionality of iPhone and iPod touch, they do it in style. Since web apps are websites designed specifically for the 3.5-inch screen, youll find the viewing experience amazing.
To browse the currently available web apps click here.
Learn to create a web app by clicking here.
Submit your web app by clicking here.