All Witnesses must have a user account for their witness server.
Hence by default all Witnesses are Steemians in that they are registered on the blockchain and have a username/account.
ALL Witnesses are block producers.
Every witness must run a server which produces blocks.
"No blocks = No blockchain"
Hence ALL witnesses are important in that they maintain the blockchain via the production of blocks.
The "TOP 20" Witnesses are the ones that make the decisions about Steem.
They are the "Highest ranked" (most voted for) witnesses.
All decisions are made by "Concensus".
Voting For Witnesses
All Steemians have the option to vote for witnesses.
This is how the Governance of Steem is set up to ensure you have your say in it.
The More Steem Power (SP) you hold the more invested interest your vote represents.
Your vote is IMPORTANT as every person who owns Steem has a vested interest in seeing Steem succeed. Your vote is your way of deciding who all among the witnesses you want to represent your best interests as a stake holder (investor in Steem).
Governance & Concensus
All changes affecting Steem must be voted in by the "TOP 20" witnesses.
These decisions are brought about via "Concensus", meaning that as per the current model in place, 17 of the TOP20 must agree + 1 Witness who does not need to be from the TOP20.
So 18 votes are required out of 21 for any decision affecting Steem & must exist prior to any changes being implemented.
The decisions are made public when a Witness implements the said changes on their server. This is considered to be a "Vote FOR" the said changes.
For more information, please refer to the FAQ section.
There is a Frequently Asked Questions - "FAQ" section in which many answers can be found to the above topics.