what is the difference between a sailboat called at 'ketch' and a 'schooner'??

Answer 1

A schooner and a ketch both have two masts, forward is the mainmast and aft is the mizzenmast. On a schooner the two masts and the sails they carry are more or less the same size/height. On a ketch the mainsail is much larger than the mizzen, but the mizzenmast is still forward of the rudder post.

If the mizzenmast is aft of the rudder post then then that would be a yawl rather than a ketch.