This is a collection of posts that explain the basic science behind anthropogenic global warming. If you know little of the science, I recommend reading them in order from the top
Has the Earth really warmed?
Hockey sticks or "Current global average temperature compared to past temperatures"
Shifting Bell Curves
Rates of Change
How we know it's being caused by CO2, part 1
A primer on the greenhouse effect
How we know it's being caused by CO2, part 2
How we know that we're the reason CO2 levels are increasing
The Carbon Cycle
How we know that global warming will continue
Global temperatures vs current carbon dioxide levels
Revisiting temperature trends since 1998
Natural cycles and global warming
Ecological consequences of global warming
What are our options?