SalesForce allows third parties to add functionality to their base service. The site where these are all listed is called SalesForce AppExchange, and they use the term app throughout. You then click a Get It Now button, you will be asked to Log in to the AppExchange Using your Salesforce credentials, and finally Install in production (or a Sandbox).
Which makes it all the more confusing that when you try to manage these apps in SalesForce, you need to go to Setup > Installed Packages. Even more confusing, there is a Manage Apps section of Setup that includes pages for Connected Apps, App Menu, and even Google Apps Settings. These are not the droids you are looking for.
Remember: install from AppExchange, but manage it under Installed Packages.