Quick answer: it is nearly always an App.
Long Answer: I have seen a lot of Comment for some apps in the Chrome Webstore that are along the lines of “This is just a link”. In some cases, the users are correct but they are missing an important point. The webstore is a channel for users to discover great Web Apps, many users who visit the Web Store will have never seen your app.
Take for example my app http://simpletodo-app.appspot.com/ and the App in the Web Store. Both load the same URL, but both are defiantly an app. There is nothing to suggest that it isn't an app, but if you check out the manifest for the code, you will see that it is really just a reference to the URL with an request for unlimitedStorage.
The Web Store doesn't have to have new applications or applications specific to Chrome; in fact I encourage you to put your existing apps in there, that is what it is designed for – to help you, the developer, to reach users more easily. I think that a lot of users when they see their favourite app in the store think that there should be a new experience for them, but you have to remember that most of the users that see this app probably haven't used it, so it IS an new experience for them.
Obviously there are some caveats, yes you can create and list “packaged apps” which have Chrome specific functionality, and yes you will notice that quite a few apps have targeted HTML5 enabled browsers (this is a good thing), but that doesn't have to be the case. I would love it if you put your existing app in there, and because you have a whole host of new users you decide to upgrade the experience to cater for more modern browsers such as Safari, Chrome, Firefox and IE9.
So if you are a developer of an app where people are saying this and you can't devote much development resource, I have one simple piece of advice that can help you: Send your users to functionality as quickly as possible. If you have a log-in page, send them to that (but make sure they can quickly sign-up). Under no circumstances send them to a default product page, if you do this, then it really is just a Bookmark Link. The user has already chosen to 'install' your app, make it feel like they are jumping into your application from the instant that they click the icon.
Even better still, if you have some time, get rid of the sign-in page, use OpenID – apps can now have a near seamless account creation and login process. Check out our article about Federated Login for some really nice examples of account creation and sign-in flows. You will see that they get users to the functionality really quickly. If you want to see this in action, check out the Diary.com application – it is a beautiful sign-in process and will show you how simple it is and the positive effect that it has for users.
As always, comments welcome! :)
About Me: Paul Kinlan
I lead the Chrome Developer Relations team at Google.
We want people to have the best experience possible on the web without having to install a native app or produce content in a walled garden.
Our team tries to make it easier for developers to build on the web by supporting every Chrome release, creating great content to support developers on web.dev, contributing to MDN, helping to improve browser compatibility, and some of the best developer tools like Lighthouse, Workbox, Squoosh to name just a few.