When you're in a conference room and need to project, connect your PC using one of the cable connectors in the room, press the Windows logo key + P, and then choose one of the four options:
PC screen only. You'll see everything on your PC. (When you're connected to a wireless projector, this option changes to Disconnect.)
Duplicate. You'll see the same things on both screens.
Extend. You'll see everything across both screens, and you can drag and move items between the two.
Second screen only. You'll see everything on the connected screen. The other screen will be blank.
Then again, you might not even need a cable. If your PC and the projector both support Miracast, press the Windows logo key + P, select Connect to a wireless display, choose a projector, and you're set.
Not sure if your PC has Miracast and can be projected to? Windows will let you know. To check, select the Start button, type Connect, and then select Connect from the list of results.