this report by the IPCC is the technical summary which is a really good intro into the science behind climate change. There are plently of graphs, and key facts are highlighted which makes it quite an easy read despite its length.
http://www.ipcc.ch/ipccreports/ar4-wg1.htm
This report also by the IPCC explains the potential impacts that climate change could have on the earth.
http://www.ipcc.ch/ipccreports/ar4-wg2.htm
If you are looking for a much more simple overview, with not much detail then wikipedia has a good over view. However it is not very detailed.
http://en.wikipedia.org/wiki/Climate_change
Other useful sites i have used are:
http://www.noaa.gov/
www.climateprediction.net
if you have acess to any journals, i recommend looking at articles from the journal climatic change, but this might cost money, depends how interested you are.
http://www.springer.com/earth+sciences/meteorology/journal/10584