UDP, which stands for User Datagram Protocol, is a protocol used for sending data packets without the need for a connection between hosts, and does not ensure the reliability of delivery, ordering of packets, or perform error checking. This characteristic makes UDP suitable for applications where speed is more critical than reliability, such as streaming or gaming. In contrast, TCP (Transmission Control Protocol) is designed for reliable communication and requires a connection, while ICMP (Internet Control Message Protocol) is used for sending control messages, not general data transfer. SSL (Secure Sockets Layer) is a protocol used for encryption, not for the basic transmission of data packets.