Session Initiation Protocol (SIP) is a communication protocol used to initiate, maintain, and terminate real-time sessions that involve video, voice, and messaging applications. It is widely used in internet telephony and is essential for establishing connections between different devices and networks. SIP works by sending messages between endpoints, such as VoIP phones or video conferencing systems, to set up and manage calls. It can handle multiple participants in a session and supports features like call transfer and hold. SIP is an important part of modern communication systems, enabling seamless interaction across various platforms.