Webapps is a new feature of Unity in Ubuntu 12.10 Quantal Quetzal which allows you to integrate some websites, such as Facebook or Reddit, with the Ubuntu desktop. Those Webapps are installed like native apps in your system and they can be pinned to launcher, used with messaging menu, sound menu, HUD etc. When you visit a site that has Webapps support you will be prompted to install it like this:
If you want to uninstall it here is what you have to do. First open Ubuntu Software Center and check if there is a package installed for this Webapp.
Next open dconf editor and go to com > canonical > unity > webapps. In allowed-domains, remove the entry which you don't want.
Finally, go to ~/.local/share/applications and remove the .desktop file for the webapp you would like to uninstall.
This isn't the fastest way to go. I think there should be an option to uninstall the Webapp just like you installed it in the first place.
Are you using any Webapps?