Who are "the establishment"?

I would class the establishment as anyone who does well out of the current social and economic system.

For all such folk there is a strong incentive to maintain the status quo that delivers such fine results for them.

On another level, the establishment are those people and groups who have control of the "public mind". 

These groups dictate what is thought of as real and what is dismissed as hooey by the average person.

Members of the establishment don't agree on everything, but they have many areas of shared mutual interest, which they protect collaboratively, and also enemies who threaten their business or status, and they target these in a similar collaborative way.