It started with CO2 emissions. By itself it warmed the environment (a little bit) but more importantly it kickstarted a positive feedback loop with water vapor, which is also a pretty potent greenhouse gas (albeit not to the extent of CO2 when comparing it molecule by molecule). Warmer temps can hold more water vapor, so the water vapor traps more heat, and the even warmer air can hold even more water vapor, so the additional water vapor traps even more heat, etc., etc., there's your positive feedback loop (also ice melting = less radiational energy reflected = warmer temps, another positive feedback loop). So with the increasing amount of CO2 emissions every year this loop only increases in rate over time.
I don't know too much about methane and CFCS so i didn't mention them. It would be much appreciated to tell me more about those greenhouse gases.