You are talking about SIP and H.323. I do not know what the difference is - which one should I choose?
SIP and H.323 are two different protocols used in voice over IP (VoIP i.e. audio communication over the internet). If you want to call Windows Messenger users, use SIP. If you want to call Netmeeting users, use H.323. If you do not know what protocol to use, use SIP. The URL prefix determines what protocol will be used, e.g. sip: or h323: (or callto:).
How can I get a SIP address?
A SIP address is a way to be reachable and to reach people. You can compare it to an email address. You can sign up for a free account on http://ekiga.net. This will give you a unique SIP address that you can give to your friends so that they can contact you. An example of a SIP address is sip:firstname.lastname@example.org.