Office Click-to-Run installs required Office functionality with streaming techology which is compared to watching a video: it is not necessary to have downloaded the entire movie to start watching it. Same thing with the application which will be downloaded as the user is using it.
So, Office Click-to-Run is not a new Office prodct, rather, it's a new method of delivering and updating the product.
Click-to-Run uses or relies on virtualization technology.
A Click-to-Run application executes in a virtual application environment (on the local machine). So, the modifications within this virtual environment don't interfere with the surrounding «real» components on a computer.
There might be a (virtual) Q:\ drive which is the virtual file system that is used by Click-to-Run.
Office Click-to-Run was introduced with Office 2013.