Car shuts off

Over the past 3 days my car has losing power- at first just slightly sputtering on the first day, then today, i drive about 1/4 mile and my car will just turn off. I will restart the car and it will drive for a brief time and shut off again. The more i drive the car, the more quickly it seems to just shut off. I have a 2 year old battery and just cleaned the battery posts and recently changed spark plugs. Do you have any suggestions on why it just shuts off??