What is the difference between Modbus, Modbus Plus, and ModbusTCP/IP?

Modbus is a serial master/slave protocol. Modbus can run over any serial type media (copper, fiber, wireless, modem, etc..)

Modbus Plus is a peer-to-peer protocol which runs at 1 mbs. The Modbus Plus protocol specifies the software layer as well as the hardware layer. Proprietary cabling and terminators must be used with a Modbus Plus network.

ModbusTCP/IP is an EtherNet based protocol. ProSoft Technology's ModbusTCP/IP products act in a master/slave capability.