In my experience, most apps on the Chrome store are what Google calls "Hosted Apps": a link plus metadata and an icon [0]. They also provide "Packaged Apps" [1], which are downloaded and run locally as you describe, but this style appears to be much less popular with developers.
0: http://code.google.com/chrome/apps/docs/developers_guide.htm...
1: http://code.google.com/chrome/extensions/apps.html