From VoIP.ms Wiki
(Just using this page as a scratchpad for now. Below I'm working on rewording SIP URI.)
A SIP URI describes an Internet address that can receive SIP calls, much the same way a phone number acts as an "address" for a traditional phone. It resembles an e-mail address:
In this example, sip.example.com is the domain name of a SIP server (it could also be an IP address), and 12345678 is an assigned account on that server. Though a SIP URI usually has a numeric account ID, it could just as easily be a name:
SIP calls are conventionally made via TCP port 5060. If a SIP server takes connections on a different port, that must be included in the URI, like so:
Here the URI indicates the same SIP server and account, but also specifies that SIP connections must be made on port 5061.