Web apps are basically launchers on your system, along with some code that is run in your browser to provide notifications and other system integration features.
The web apps you have installed are stored in
~/.local/share/applications and in some cases in
/usr/share/applications (this is in the case of applications you install using the Ubuntu Software Centre). You can search for Webapps from the Dash and add them to your launcher as well, but you cannot really control them as such by default.
There is an application called Unsettings which allows you to enable and disable specific web applications, or you can do so manually by removing their