An updated version of the first demo. This would probably have been the demo that was supposed to be released instead of the one back in november of last year if things weren't rushed.
Everything from the previous demo applies here in terms of file size and gameplay.
Extract the file into your half life folder, ex:"...\Steam\steamapps\common\Half-Life\" and restart steam.
There is still room for change, so things may be updated in the future for the final release.