Can someone explain after hours trading and how it works/is carried out?

I was wondering how after hours trading works. I use RH and I noticed the percentages and prices change during non market hours, how exactly/who exactly is able to control this?

Sorry for the dumb question, just a bit confused and would like to learn.

Submitted July 10, 2016 at 07:41PM by ignore_what_i_say http://ift.tt/29xwIEX