The following is taken from the ODVA.org publication PUB00138R2:
"EtherNet/IP has two types of messaging connections. Implicit and Explicit. Implicit messages are point-to-point relationships that are established to facilitate request-response transactions between two nodes. These connections are general purpose in nature and can be used to reach any network accessible item within a device. Explicit messaging connections utilize TCP/IP services to move messages across Ethernet."
"Implicit (I/O data) connections are established to move application-specific I/O data at regular intervals. These connections are often set up as one-to-many relationships in order to take full advantage of the producer-consumer multicast model. Implicit messaging uses UDP/IP resources to make multicast data transfers over Ethernet a reality."