Damn, there's a lot of confusion, contradicting statements, idiocy, corruption, and lolwut around this topic. Now, I've done an Environmental Engineering course. Hence as an expert who is expertly placed, you can trust me to wade through this muck and give you the right answer.
Let's start from the top. The Greenhouse Effect. No, you won't need Wikipedia for this one. It's simple.
Most things in this world are opaque. Which means they don't allow light to pass through them. The wavelengths of visible light they don't absorb gives them their color.
Gases are mostly transparent due to their low density. This is different from colourless, which means that most gases do not absorb visible light. But when you have a huge amount of air, such as an atmosphere, "transparent" becomes irrelevant.
Different gases in the atmosphere absorb different wavelengths of light. What happens when they absorb it? They re-emit it in a different direction, and often with a different wavelength. Water Vapour, CO2, Methane, Ozone are the major players in this. They absorb visible light and infrared light, and re-emit it as infrared. This is called the Greenhouse effect.
This is a good thing. Without the greenhouse effect, we could not survive. Temperatures on our planet would be extreme, with the average temperature being sub-zero. The major contributor to our survival is water vapour, which keeps a large amount of thermal radiation around Earth.
Disaster strikes when someone decides to pump a bazillion litres of CO2 into the atmosphere every year. What does this do? It's complicated. Complicated enough that we don't know for sure the dynamics of everything it does. It's not as simple as rocking a cradle. The global climate is a chaotic system. Even tiny changes in the initial conditions magnify exponentially and change the result completely. This is why BBC doesn't show more than a week's forecast in weather.
We never just say "OK, T°C increase in global temperature in X years" "Polar ice caps will melt in Y years". That's what uninformed news outlets say. They try to dumb the results and predictions down for the general public, and in the process make them incomplete, inaccurate, or wrong.
This is also why scientists' predictions concerning climate change seem to be "wrong" most of the time. This leads the naive public to assume that we're talking out of our collective arses, and that Climate Change is bogus. And when we try to explain what we meant, people go "NYA NYA NYA, YOU'RE WRONG, NOW STOP MAKING EXCUSES AND SHUT UP". And that's what we do. Because we know we're right. That's the thing about science. It doesn't matter what you think, the truth will stay the truth.
Here's the truth. We don't know exactly what's happening (yes dears, present tense) thanks to our shortsightedness. But we're all bloody damn sure about one thing. Whatever's happening isn't good for us. That's why it's called "Anthropogenic Climate Change" (man-made and not-good-for-man).
Now that I'm done being all dramatic and apocalyptic, I notice I forgot to mention all that stuff about Albedo, Positive feedback loops, Quasi-static equilibrium, cyclic climate change, frog in boiling water, etc. Oh well. Maybe if everyone paid attention in college, I wouldn't have to start from the top and get bogged down in the basics before getting to the interesting stuff.