Blackjack is not one game. Spanish 21 removes the tens. Free Bet Blackjack gives the player free doubles and splits, then takes it back with a dealer 22 push. European Blackjack deals the dealer a single card first. Pontoon changes the names, the payouts and who wins a tie. The rules look small on paper, and they move the house edge a lot.
The site puts those games side by side. Each variant page sets out the rules in plain language, shows the house edge worked out from those rules, gives a strategy chart for the game, and offers a play-money table that follows the same rules. You can start at the home page, or go to Spanish 21, Free Bet Blackjack, European Blackjack or Pontoon.
The tables are for learning. You can make mistakes at them, see what the chart says, and find out how a rule changes the result, with nothing at stake.