Real Climate is a site maintained by working climate scientists. It is essentially a "one-stop" page for climate knowledge. They provide up-to-date information on climate legislation and detailed explanations on publications and climate research. We highly recommend you peruse the site.

A few key takeaways from their "beginners" section:

1) Earth's climate is complex, and consists of multiple spheres (land, sea, ice, atmosphere).

2) Climate and weather are two different subjects. Weather refers to short-term events and variability, while climate refers to long-term trends. It can be very difficult to predict weather patterns because they are erratic, but climate change is more predictable and easier to model. 

3) Without the greenhouse effect, life as we know it could not exist. The greenhouse effect traps energy in the atmosphere and reradiates it back to the earth's surface. 

4) Carbon dioxide is a greenhouse gas. Thus, higher concentrations of CO₂ (all things being equal) will alter the transfer of atmospheric energy and warm the planet. 

5) Humans have been responsible for increasing greenhouse gas emissions.