Witnesses are responsible for producing and verifying blocks on the STEEM blockchain.
Additionally, witnesses influence what happens during a "hardfork".
Witnesses are chosen using a system called Delegated Proof Of Stake (DPOS).
DPOS simply means, anyone who holds STEEM has 30 witness votes that they can cast to help influence who creates new blocks.
For a more detailed description of what a witness does:
https://steemit.com/witness-category/@someguy123/seriously-what-is-a-witness-why-should-i-care-how-do-i-become-one-answer
It is important to have witnesses who are trustworthy and active in the community.
themarkymark engages regularly in chat and genuinely cares about the platform. He is always willing to help people troubleshoot issues they may encounter and gives regular witness updates on his blog.
Here is his latest witness update: https://steemit.com/witness-update/@themarkymark/witness-update-themarkymark
To vote for themarkymark:
https://steemit.com/~witnesses
Use your witness votes, they matter!!
themarkymark is also the "brain" behind the popular booster 
buildawhale has recently increased their steempower and is now producing a daily curation digest. Any post that you submit with a bid is considered for the daily digest. For a small fee you can also submit a sponsored post to appear in the curation digest.
Latest curation digest:
https://steemit.com/buildawhale/@buildawhale/buildawhale-curation-digest-9-30-17
To find me or themarkymark, come chat with us in whaleshares discord!
Other witnesses I endorse:
