You can view demos from any Half-Life mod or game by using SK Player by bds or merely using Half-Life to launch the demo through the console. I recommend SK Player as it has proven to be the best method for viewing demos in Half-Life. A demo is a file which captures the current netsteam after the record command is used. You can play demos to view CS matches between good clans and develop strategies to become better, or to get behind the eyes of your favorite players and teams.
You can record your own game demo by bringing up the console and typing the command "record [demoname].dem" (without the []'s) to immediatly begin the recording process. When you are through recording what you want to be in your demo, you will type "stop" in the console to stop the demo recording. Now a [demoname].dem file is in your Steam's Counter-Strike directory.
To play a demo without the aid of a third party demoplayer such as SK Player you must launch Counter-Strike and use the console command "playdemo" or "viewdemo".
Command: playdemo
» Plays a demo
» example - playdemo my_first_demo
Command: viewdemo
» Plays a demo using the new viewdemo GUI
» Allows for fast-forwarding, rewinding, and pausing
» Speed up and Slow motion
» Warning: This method is still somewhat experimental and you may experience problems
» example - viewdemo my_first_demo
ViewDemo Control Panel
Although you can use the Half-Life console commands to play demos, many factors, such as a difference between your resolution and the demo's, can quickly sour the experience. SK Player is the best way to go when it comes to playing demos with reliability and quality.
A final note on Half-Life demos, they are rarely perfect. Don't be surprised if demos you download fail to work. Demos recorded on older versions of Half-Life or Counter-Strike will more than likely fail to play correctly on the latest version of the game. Demo bugs are common, even when recorded and viewed with the latest version of the game. If you must view older demos you will need the old style HL/CS client version 1.5 (non steam). With this, you can use SK Player's "old playback fix" which in many cases will allow you to watch demos recorded on CS versions 1.1 - 1.5. If this does not work, or you need to watch even older demos, your only hope is downloading and installing the older versions of both Half-Life and CS, which are hard to find.
The above are copied and pasted from Shauq