http://steamcommunity.com/tradingcards/faq/
So basically, you get trading cards by playing games / buying off other people. Once you get the whole pack, you get 'booster packs' which help with getting exp for your steam level. And steam level just gives you nifty features on steam such as customizing your profile more or increasing the max no. of friends you can have on your list.
In other words, not that useful lol.