As far as comparing IP and SIP, it's not so much a matter that there's a "difference" between IP trunks and SIP, but that SIP is actually a set of strict rules and standards (protocol) for the specific treatment of IP trunking. In addition to SIP (Session Initiation Protocol), some of the other VoIP protocols include H.323, H.248, and Media Gateway Control Protocol (MGCP). Like you, I'm learning the IP side of telephony myself, and I think SIP is easiest explained
here. If I'm not mistaken (and someone please correct me if I'm wrong): SIP also provides for IP trunks to directly interface to SIP-enabled switches and IP/hybrid PBXs without the requirement of an externally installed gateway device (such as a Lucent I-Merge Gateway, which is an externaly rack-mounted appliance that provides interface between TDM and IP trunks.)