When I go to MyEtherWallet.com, I found a list of Ethereum networks like picture below. Could someone explain how many Ethereum networks actually exist and what is the difference between them? Thanks
The official ethereum network is
ETH . The 4 first instances of ETH are simply different nodes you can connect to.
ETC is a fork of the official ethereum blockchain (ETC and ETH have the same common ancestor)
As anyone can run a slightly tweaked Ethereum network, others networks are non-official (and mostly for testing purposes) networks implementing different logics (ex: POA is a network implementing Proof Of Authority as a form of verification...).
I believe those are nodes which connect to a network. The first word is the network name and the stuff in parentheses
() is the node address/name. The address/name seems to often also indicate if it's some provider (like Infura).
I'm unsure how the list is populated, but you are probably able to connect to whichever of those nodes.
List of test networks can be found here: Comparison of the different TestNets . But about the other networks - maybe some other more private networks.