There are a few different ways to use a Chromecast on a PC, and they all involve using the Google Chrome web browser.
By using Chrome, you can stream content from a single tab or stream content from a file on your computer.
To make your selection, open Chrome and go to More> Cast. A new window will appear, displaying all the Chromecast devices, video and audio, on your network. Click on a device to select it, and select "Sources" to choose whether you want to stream a tab, a file, or your desktop.
If you cast a single Chrome tab or choose to cast a file, you can continue to use other Chrome tabs and the rest of your computer without affecting the cast and without revealing the content on your screen to people viewing the cast output.
The only requirement is that you leave Chrome open and running in the background, even if you are working in other applications.
Conversely, if you decide to stream your entire desktop, whatever is on your computer screen will also show up at the streaming destination. Before choosing this option, make sure you do not have confidential information on your computer that you do not want to make public.
Once again, you need to leave Chrome running for it to continue streaming.