What is Steemit and how can I use it for knowledge as well as to earn money?

Can anyone please tell me how can I use Steemit to earn money as well as gain knowledge.

I have been using Steemit for almost 2 months but yet I am not fully aware of Steem powers, Steem dollars and buy wallet etc etc. such terms.

Can anyone please elaborate, it will be so helpful for me.

Thank you.