This talk at Meet Magento New York 2014 is Varnish Cache and its usage in the real world by Ivan Chepurnyi. Varnish is a way to accelerate sites such as Magento websites by caching as much content as possible.
Varnish is controlled by VCL - the Varnish configuration language.
vcl_recv is the entry point for a customer request, and you can detect a customer segment based on cookies and serve a specific type of content for that segment.
vcl_hash builds the cache hash for a page - it gives the possibility to serve different content based on customer device type, detected customer segment, etc.
Varnish is all about communication via headers - specify cache TTL from Magento in headers.
Cookies connect dynamic actions, shopping cart, etc., in Magento and Varnish.
Browser storage should be used to minimize AJAX calls.
There's a framework that already caches basic pages in Magento, and you can easily add your own page type. Ivan has released the code on GitHub at https://github.com/EcomDev/EcomDev_Varnish