The Ethereum main network, and its corresponding test networks, are all open for public use. All transactions made on the networks are 100% transparent, and anybody with access to the network—which is potentially everybody—can see and access all data.
The users of these networks don't necessarily know each other: they are mutually distrustful parties who must assume that the other participants in the network are dishonest. For this reason, public blockchain networks require a way for such parties to reach an agreement without needing to trust each other, and this is where decentralized consensus algorithms, such as proof of work (PoW), come into the picture.
There are cases, however, where transacting parties in a network are more trusting of each other, and where the use of a blockchain isn't solely centered on allowing mutually...