In my opinion the deep state is a network of shadowy influential operatives--I say shadowy not because some are not well known but because they work out of view of any scrutiny--whose purpose is to direct, engineer, aid and abet a specific agenda and undermine or destroy anything and anybody who is at cross purposes with that agenda.
They work hand in hand with whatever government staffers or employees that are sympathetic to their cause, almost certainly with some members of congress, with big money donors, and many with influence in media, social media, education, scientific institutions, big business, and most likely entertainment and religion.
Loosely defined, it is basically 'the swamp.'
Certainly not all associated with any of those things, not even most of those who share the agenda, are 'deep state.' But I believe it exists and, in its current form, is anti-American, anti-constitutional restraints, anti-American values, pro one world government and total control of the people.