Ask HN: Who are the major players in the digital advertising world?

I've recently gotten into the digital advertising to advertise my own business, and I was wondering who the big "players" in this world are. A few are obvious:

* There's advertisers like me, who pay money for advertising.

* There's large content-based sites, from Google, Youtube, Facebook, Amazon, to smaller ones like Quora and reddit, where you can pay to get your ads placed directly

* There's third party sites (not to mention things like games) that run ads through display ad networks.

* There's an entire world of affiliate marketing where people try to sell other people's products for a cut.

* There's "influencer" based marketing, where major brands pay e-celebs to show their products, though I guess that's more traditional than "digital" marketing...

Who else is involved? I feel, given the sheer size and complexity of the advertising space, there must be more companies out there. Is there a secondary market for google adwords? Are there 'hedgefund' like companies that make proprietary bidding algorithms?