First to use you plug in the headset to the microphone (usually pink)socket on you PC and the other headset lead to the external speaker/phones (usually green) socket on your pc. Webcam is usually a USB connection and may come with software to load onto PC first. If using Win XP should detect new hardware and install it.
If you use these to chat via the PC to other pc users there is no cost to you over your existing PC to isp connection charge (broadband or dial up).
Programs you can use to make use of it usually require you to have others using the same system but at additional cost you can connect to landline phones as well. Software available includes (but by no means exclusively):
Windows Messenger Live
It's a whole new world of telephony.