The "greenhouse effect" is naturally occuring. We would actually die if we didn't have it. What it means is that the gases in our atmosphere trap some of the heat from the Sun and the rest escapes back into space. If this didn't happen, we would be warm during the day, and freeze to death at night because all of the heat would have escaped.
I do think humans have altered the greenhouse effect to function in a way that is not natural. Because we are unnaturally putting large amounts of gases into the atmosphere (like CO2), it in increasing the amount of heat that is being trapped. That is what is causing global warming, and it is definitely a man-made problem. Are there other natural factors that might contribute? Of course! But, it is us that is altering the ratio of gases in the atmosphere and causing most of the problem.
And that is your science lessong for today! :)