^^ Both are just mods of half-life 1 & 2
You need half-life 1 to be able to play the original Counter Strike.
You need half-life 2 to be able to playce Counter Strike:Source.
I know for half-life 1, you need to download the gaming client Steam which will then download Counter-strike onto your computer. The reason you need half-life 1 is because of the CD-key... that's the only reason, however Valve deletes stolen cd-keys and bans them, so you can't just bootleg it.
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