Steam trading cards help

They only drop when you play a steam game with cards. They drop each 15 minutes of game time, and there's a limit in how many cards can be dropped per game.

From games you bought, you get half of the cards automatically as you are playing. The other half, you have to buy on the market or trade with other Steam users.

For F2P games, special rules apply (you have to spend certain amount of real money in order to get your cards).
I don't know how exactly it works, but this FAQ should help :)

i wanted to know because me and my cousin have bought 2 copies of euro truck simulator 2 and while we were playing multiplayer i got some cards and he got diffrent cards reather than me. Thank you guys for the help