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Hi @karensuestudios, witnesses are a group of Steem operators (anyone around the world can choose to be) that support the entire Steem network's computing and data resources. In essence, there's no one single party owning the entire network, which is why it's uncensorable. Witnesses are there to keep the network's ledger truthful, so users can vote on the witnesses they trust to get the job done.

Thank you for the explanation. How does a witness support the entire system?

Running software on their servers. This is a piece from 11 months ago, might have some outdated info, but it's pretty much what it is. It's a pretty awesome piece of tech :)
https://steemit.com/steemit-guides/@pfunk/a-full-steemit-user-s-guide-to-steem-witnesses