The single player is different from Condition-Zero. The single player is called Counter-Strike: Source which comes with CZ if you were to buy it. CZ however is just a HL mod... Steam trys to make you buy a cd key for it, but you can still use it without one.
The origional CS team doesn't work on CS anymore. They started making 1.6, and Valve basicly took it into their own hands. Because of legal issues, Valve made CZ which is their own mod. That is why all the weapon names changed.