Hello! Project Database (or Hello! Database as which it is also known as) is an information database covering Hello! Project. The site is quite comprehensive on the subject.
In short, Hello! Project is a collection of J-Pop groups and artists, probably best known for Morning Musume and Aya Matsuura. For a more detailed summary of the Hello! Project, here's a link to a wiki with additional information.
If you are interested, go ahead and enter the site.