When you have a close coupled cistern - where the cistern sits directly on the back of the WC pan - and you get water leaking onto the floor particularly when flushing - the problem is usually caused by the low pressure seal between pan and cistern - it's called a doughnut washer because of it's shape.
You can often fix the leak by just tightening the compression bolts - two wing nuts pointing downwards - one each side at the very back of the pan.
http://www.toiletspares.co.uk/images/st/st_CC_kit_doughnut_washer_new.gif
If the water inlet pipe is leaking all the time - try tightening it up - clockwise.
If this does not work - call a plumber
Good luck...
Peccavi
July 2010