What is the difference between MODBUS, TCP-IP and Ethernet?


Modbus is a data communications protocol originally published by Modicon (now Schneider Electric) in 1979 for use with its programmable logic controllers (PLCs).
The Transmission Control Protocol (TCP) is one of the main protocols of the Internet protocol suite.
Ethernet is a family of wired computer networking technologies commonly used in local area networks.

So, all these three have something to do with communication, but on a different level :) "Ethernet" defines physical/link level. TCP/IP defines network-level protocol. And Modbus is application-level protocol (originally, for serial lines, although there is also Modbus/TCP version nowadays .. and often, serial communication is converted to TCP datastream (or UDP packets) and travels via networks :)

Wikipedia is your friend. Google is another one.