Capture the Flag is a popular modification developed by David "Zoid" Kirsch. The most popular type of ctf in qw is Threewave CTF.
You must install this package to play on servers using the custom ThreeWave flag models and levels for version 4.0. You must also have the file 3wctf301.zip (which contains pak1.pak).
Why not just pack all the pak files into one single file? That should save me a bit of time.