Yes, apps can be triggered from the browser. Clicking a link that goes to Google Maps opens the app, and YouTube embeds play the video full screen without leaving the browser. 3rd party apps can also register, either with a custom url (app://), or by a URL rule (http://app.com/redirect/to/app). Apple's own MobileMe Gallery app is a recent example of this. If I try to visit http://gallery.me.com/user, it opens the app.