SIP (Session Initiated Protocol) is simply the protocol that powers VoIP. It was developed by the IETF in 1996 and has since evolved to become incredibly flexible, reliable and capable of handling pretty much any call feature that any phone system has ever had to handle - this is likely because of it's huge adoption among small & medium size businesses, roughly 16% of SMB's have SIP phone systems as of 2014. The protocol is highly technical and meant to be interfaced only with phone system engineers & developers - certainly, not the end user of the phone system. See more about SIP as a protocol on wikipedia , or if your really dedicated, take a look at the SIP protocal specifications .
If you are looking to set up your own SIP phone system, RingRoost's PBX builder offers a simple drag and drop interface that allows you to do this without any technical or system engineering experience. Using the PBX builder (a PBX is just a phone system), you can set up multiple phones, each with their own extension, voicemail, and a number of advanced call features.
Most SIP phone systems will not have a graphical user interface for setting them up, generally you need to install your own hardware / software and then program a number of setting in order to meet your business requirement. If you are looking to set up your own SIP phone system without using a graphical interface, here is a list of some great SIP software tools that you will almost certainly need.
Phone system's using SIP make up a significant percentage of the SMB communication systems market, roughly 16% of the 75 million SMB's in 2014 use SIP phone systems. More shockingly is the growth rate of the SIP phone system market, the SIP/VoIP market on the whole is growing at rate of about 13% per year. Our internal research projections of the SIP market predicts that almost 40% of SMB's will be powered by SIP phones by 2023.
There are many other communication protocols in traditional phone lines, however comparing those protocols with SIP is not really fair as SIP goes over internet and the traditional phones go over PSTN.
There are a few other communication protocols that "compete" with SIP and go over the internet. The most notable protocol is a protocal call IAX , this protocal is generally used when communication betweet two servers rather than a server and client or SIP phone. Often times SIP/IAX can work together in a large phone system, however many systems will just use one or the other for the vast majority of communication signaling. Some other communication protocols have emerged that do a simliar thing as SIP, however they are almost always from a private organization and proprietary in nature, a notable example is the Skype Protocal.