Some people still do not understand what a witness. There is indeed a lot of contradictory information about Steem as to his role and his income.
I therefore intend to explain the concept as simply and comprehensively as possible in this post.
What is a witness?
The witness plays a vital role in the proper functioning of the Steem network. You may not know it but Steemit is actually only the sophisticated interface of a large network called Steem that operates a bit like Bitcoin.
Simple definition:
A witness steem is a person who manages a witness node (which produces blocks) and publishes fluctuations in the value of the US dollar (USD) against the STEEM currency on the network.
The witnesses therefore have many tasks. The most important of these is the production of "blocks" (which in turn allow transactions to be processed). Their second role is to manage currency fluctuations and thus ensure that a STEEM dollar is still worth one US dollar (thanks to a 3.5-day exchange rate).
You may also have heard of the term " 51% attack" . Be aware that unlike Bitcoin, it is the witnesses and not the miners who have influence on the Steem network:
For example, a hard fork (or an update) can only be adopted if 67% (more than 2/3) of the best witnesses execute a particular version of Steem at the same time. Specifically, 20 witnesses alone can decide whether or not to accept a fork (for example, hard fork n ° 13 and 14 had to be reviewed and even postponed for a week because the witnesses refused to update their version. ).
In summary,
They are the main creators of blocks (in Steem, we do not speak of minors but of witness);
They continuously publish the currency fluctuations that are responsible for the operation of the Steem Dollars (SBD);
They work by consensus (by deciding whether or not to accept a hard fork );
Many witnesses , such as 
If some witnesses do not produce content or participate in any project to develop Steem, they remain very reliable ( 
What does it concern me?
Apart from the first 20 of the rankings (which produce a block every 63 seconds), a witness is paid in proportion to his rank.
But how to progress in this ranking? By getting the votes of people like YOU. And if you do not vote, other members could give their vote to witnesses who would be much less reliable or even dishonest.
How are the blocks produced?
The process is not as uniform as one might think. Blocks are produced every three seconds during rounds . These are composed at most of 21 blocks.
Each round , the blocks are distributed as follows:
20 blocks are awarded to the 20 witnesses who have collected the most votes (the order within a round is random to avoid that some people deliberately miss a block);
The last block is assigned to a witness who is not in the top 20 but whose selection process is influenced by their rank.
The ranking is based on the total number of votes cast by their constituents.
You can follow the schedule live on https://steem.bitcoiner.me/schedule/ .
If you want to learn more about this topic, you can go to this article (be careful, it is quite complex) written by the technical director of Steemit (Ed .: Dan Larimer is no longer CTO Steemit, he resigned in March 2017) 
https://steemit.com/steem/@dantheman/steem-witness-scheduling-algorithm
How often are blocks allocated (and how much do they yield)?
As mentioned above, the witnesses are paid in proportion to their ranking rank. A witness ranked in 30th position is capable of producing four blocks per hour while the 50th in the rankings can barely create one.
The rewards for creating a block are given in the form of Steem Power (be careful, not a STEEM or a SBD), which means that the witnesses have only very low incomes for two years.
(Note: a year ago, the procedure of Power Down took two years, or 104 weeks, today it only takes 13!).
@ someguy123 on SteemDB
I currently occupy the 39th position and I produce between two and three blocks per hour. According to the calculation (2 x 24) x 31 = 1488, I therefore earn about 1,500 Steem Power (SP) per month.
What about the top 20? What do they earn?
Currently, the top 20 witnesses are rewarded with 0.189 Steem Power (SP) per block, while the others receive five times that amount, or 0.945 SP.
For example, a classed witness 50 generates about twenty blocks a day, that is to say:
20 blocks x 30 dx 0.945 SP = 567 SP / month (or 6804 SP / yr)
While a top 20 witness is rewarded for 1 block every 63 seconds, that is to say:
(60 sx 60 mx 24 h) / 63 s = 1371 blocks / day
Their monthly income thus amounts to:
1371 blocks x 0.189 SP / block x 30 j = 7773 SP / month (ie 93276 SP / year)
Can everyone start as a witness?
In summary, the answer is yes.
But before becoming a witness, you must meet several conditions:
Have a powerful (and reliable) server that can host a witness node;
Owning a second server with a good bandwidth that can host a seed STEEM node (similar to the Bitcoin node and which allows to distribute the blocks);
Create a signature key and publish your desire to become a witness through cli_wallet;
Accurately disseminate STEEM / US dollar currency fluctuations and update them at least once a day;
Have technical skills, that is, be able to understand most of the Steem concepts and quickly solve server problems;
Prove to the community that you would make a good candidate witness.
Some of these conditions are not necessary (especially seed nodes ) while others are only useful if you are nearing the top 20 (eg STEEM / USD fluctuations). However, it is highly recommended to fill them all out if you want to become an influential witness.
If you want to embark on the adventure, I suggest you join #witness on STEEMIT.CHAT and ask for more help.