If you're unaware, they run what are essentially slot machines inside there games.
A super simple breakdown is as follows:
1.) play game for long enough, get item drops ( used to be 4 total per week in csgo ) ( either random gun worth $0.20 ~ $10.00 or case ) Cases are locked ( must be unlocked to get goodie ). The keys to open are sold by valve for $2.50 a pop or by people like myself for $2.00 ( I want to cash out to USD so I buy directly from valve ).
2.) When you unlock a case you play a literal slot machine, there are crap items worth pennies all the way up to $xx,000 knives that you can win.
A big difference in valves case is that the reward rates are known to a pretty accurate number and you don't have to gamble to win in the system. As long as you play you get item drops that you can sell for real money ( rather than gambling them ). Wth the gambling sites, it's much more like betting on football but the house is probably screwing a bunch of people.