SIP is a protocol used in VoIP communications that allow users to make voice and video calls. A SIP client is a program that you install on your computer or mobile device. It contains softphone functionalities and some other features and provides an interface for you to communicate.
There are different types of SIP clients. The most common ones are the applications offered free by VoIP service providers, to use with their VoIP services. Some of them support SIP. But you have clients that are built for SIP and do not depend on any service. You can use them with any SIP account, and even within a PBX environment. Read more about SIP and it’s benefits.