TCP is connection-oriented; once a connection is established, data can be sent bidirectionally, and TCP provides guaranteed delivery packets. UDP is a simpler, connectionless protocol that provides a network without the overhead of a reliability mechanism.
Some vSphere-related services are based on TCP or UDP, depending on the type of service.
For more information about UDP, see https://tools.ietf.org/html/rfc768.
For more information about TCP, see https://tools.ietf.org/html/rfc793.
For more information about TCP, see https://tools.ietf.org/html/rfc793.