An easy explanation:-
http://www.scientificamerican.com/video.cfm?id=how-does-radiocarbon-dating-work--i2012-11-30&WT.mc_id=SA_CAT_EVO_20121203-Most accurate at 3-4 half-lives. Roughly 90% accurate. 4-5 other isotopes are also used and agreement is within 15%.