SIP offers a discovery capability. IF a user wants to initiate a session with another user, SIP must discover the current host(s) at which the destination user is reachable. This dsicovery process is frequently accomplished by SIP network elements such as proxy servers and redirect servers which are responsible for receiving a request, determining where to send it based on knowledge of the location of the user. and then sending it there. To do this, SIP network elements consult an abstract service known as a location service, which provides address bindings for a particular domain. These address bindings map an incoming SIP or SIPS URI, sip:bob@biloxi.com for e.g. to one or more URIs that are somehow closer to the desired user agents at which the desired recipients is currently residing.
Registration creates bindings in a location service for a particular domain that associates an address-of-record URI with one or more contact addresses. Thus when a proxy for that domain receives a request whose Request-URI matches the address of record, the proxy will forward the request to the contact addresses registered to that address-of-record.
There are many ways by which the contents of the location service can be established. One way is administratively. For e.g. IF bob is known to be a member of engineering department though access to a corporate database. However, SIP provides a mechanism for a UA to create a binding explicitly. This mechanism is known as registration.
Registration entails sending a REGISTER request to a special type of UA known as registrar. A registrar acts as the front end to the location service for a domain, reading and writing mappings based on the contents of REGISTER requests. This location service is then typically consulted by proxy server that is responsible for routing requests for that domain.
To note, Registrar and Proxy can be a single device. For explanatory purposes, it most of the times depicted as two different entities. It can also be noted that UAs may reach the Registrar via proxy servers.
References:
https://www.ietf.org/rfc/rfc3261.txt Section 10
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