What exactly is the "Greenhouse Effect?" First, the term "Greenhouse" is really a misnomer. A greenhouse is a building made of glass on all sides and the roof. The glass traps in heat as it acts as a barrier much like a roof in our own homes. On our planet, we do not have such a barrier - rather the atmosphere is open to the outer space that surrounds it. A Greenhouse gas, however, is one that can absorb long wave (infrared) radiation for a period of time before re-releasing it. Thus, these gases slow the escaping heat during the time between sunset and sunrise.
Greenhouse gases are very important to life as we know it. If these did not exist, our day to night temperatures would fluctuate wildly as they do on a planet like Mars. On Mars, it is not uncommon to see temperature variations of over one hundred degrees each and every day. Thus, these gases help to moderate the temperatures on our planet and make it livable.
The most important Greenhouse gas is water vapor and it accounts for over 90% of the effect. Carbon dioxide is the second most important one and accounts for between 4 - 8% of the effect. While the carbon dioxide in the atmosphere is evenly distributed, water vapor is not. The amount of water vapor in the atmosphere varies by location, time of year, current weather conditions, etc. This is part of the reason why more exact numbers can not be given as to the effect of each of these gases. Finally, it should be noted that there are a few other Greenhouse gases - most notably methane.