Explaining the Green screen

In class, Rebecca was explaining to us how to use edit on FINAL CUT PRO - taking out the green screen. It sounded fairly simple when she explained it. Some of our shots were hard to do because our artists hair was all over the place or sometimes the shadow in the shot was just too big. We used something called the 'Chroma key' to get the right shade of colour and then you drag whatever you want instead of the green screen on top of where the green screen was - where the base tracks are placed. ( I hope I have explained this properly)