I know this confuses many people, and I myself forget this sometimes, so I decided to make a guide to help people that need to know how to do this.
To take a screen shot:
Simply press the key right beside f12 that says "Print Screen" It may be shortened to say something like "Prt Scr" this will take a picture of exactly what is on your screen at the moment, and copy it to your clipboard.
After you've pressed the Print Screen Key, open up Microsoft paint or some other image manipulation program such as gimp, and paste it in (ctrl + c or edit>paste).
It should paste that image in as long as you haven't copied anything else to your clipboard.
To take a video:
Press you console key (~) and type /record in the console and press enter. on the bottom left you should see something that says sometime similar to:
RECORDING demo 0001: 10kb
This video will record all the conversations, including Private Messages sent to you. This video will not record what you type in the console, what you type in the text box, or any modifications you have done (things to the sight, or the fps box, etc.)
This will continue to record until you stop it, or when the round ends. If you want to stop it early, open your console again (~) and type /stoprecord and it should stop recording.
To play your video, start up et, connect to a server, disconnect, go to the menu and click the button that says "REPLAYS" then select the demo number, let the video load and watch!
You can also re-play it from the console, but I forgot how to do that (sigh) If any of you remember how, please tell me, and I will add it to this post.
There are many hacks and tricks you can do to manipulate this video, like remove player names, conversation, re-record this video as avi. If any of you guys know some commands please post them!
Thanks for reading, I hope this helps some of you guys out!