To answer your first question this explains the differences between apps and extensions (I think there’s no better way to explain their nature):
https://developers.google.com/chrome/web-store/articles/apps_vs_extensions
What do you mean with “what kind of apps”?
You can develop both hosted and packaged apps if is that what you intended, give a look at this:
https://developers.google.com/chrome/apps/docs/index
Anyway the easiest way to start would be by building an extension, how-tos and
good documentation makes it good to start with:
http://developer.chrome.com/extensions/getstarted.html