The store will be accessible through both platforms and installed applications will exist as new tabs. As you might expect there will be a number of apps for Google services, such as Gmail, Google Docs, Google Calendar, Google Reader, Google Wave, but the gallery is not limited to Google products.
For instance, you can expect apps like TweetDeck, along with games such as Plants vs. Zombies and Lego Star Wars. Developers will be allowed to upload their software to the store on their own, and the marketplace will include a built-in payment system.
The Chrome Web Store should be added to Google's browser later this year, and will presumably be live in time for Chrome OS' debut.