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A witness is responsible for running a high end server which produces blocks and processes the transactions, posts and comments on the steem blockchain.

They are also responsible for publishing a steem and sbd price feed to the blockchain.
When there is a hardfork the witnesses are responsible for deciding on the fork.
Also a lot of witnesses do other things for the steemit community and contribute in a positive way.
I am currently running 2 other steem related websites, http://howtosteemit.com/ and https://steemfriends.com/ (bear in mind they are still in progress)
i also have other projects in the pipeline

being a witness is not an easy task and as yet my server rental costs me more than i get in return from running the server.
A witness gets to produce more blocks by being voted higher in the block production list.

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