What is a SIP server?

SIP stands for session initiation protocol. These servers are used for serving voice, audio and other communications over internet protocol. Network elements of  SIP server can be found in this link.

http://en.wikipedia.org/wiki/Session_Initiation_Protocol#Network_elements

web server vs web container vs application server

web server : is something that is capable of handling http requests (recieving http requesting, processing them, creating http responses and sending them to the clients). Apache Web Server

web container : is a J2EE compliant implementation which provides an environment for the Servlets and JSPs to run. Tomcat typically a web container

application server : is a complete environment for running business components. It contains the web server capabilities as well as web container capabilities. eg: IBM WebSphere, Oracle Application Server