If you have a steam account and you download a game how do you download it onto another computer

If you have another computer you can install steam onto that one and re download the game but it will not allow you to duplicate the game because thats illigal copy and violates your agreement with steam. So only one person can have the game on one machine so if your buddy wants to play with you he will have to set up his own steam and get the game. You can purchase the game and send it to him as a gift on steam one of the cool options they allow to share a game but you still have to pay for it unless its free