Visit the website....
Search on google for steam (1st one on the list)!
It has a brief explanation menu for what it is and why it's usefull!
In my own words id say its a large gaming community of which you can play other candinates in a huge majority of games "Of which you buy"
were talking call of duty,Counter strike,half life,left4dead...
etc....
For more info visit there website.... download it for free...!