Transport layer protocols are a type of communication protocol that is used to provide host-to-host communication services for applications. They are a part of the transport layer in the seven-layer OSI (Open Systems Interconnection) model for network protocols. The two most common transport layer protocols are TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).

TCP (Transmission Control Protocol): TCP is a connection-oriented protocol, which means it establishes a connection between the sender and receiver before transmitting data. It provides reliable, ordered, and error-checked delivery of a stream of bytes. TCP is used when the reliability of data transmission is crucial, such as in web browsing, email, file transfers, etc.

UDP (User Datagram Protocol): Unlike TCP, UDP is a connectionless protocol. It sends data packets without establishing a connection, which makes it faster but less reliable than TCP. There's no guarantee that the data sent will reach its destination or that it will arrive in the order it was sent. UDP is used in applications where speed is more important than reliability, such as in video streaming, online gaming, and voice over IP (VoIP).

In summary, transport layer protocols like TCP and UDP play a crucial role in data communication. They ensure that data is delivered from one device to another over a network, either reliably with TCP or quickly with UDP.