What is the difference between a floribunda and a grandiflora rose?

Floribunda roses have large clusters of flowers,often with shortish stems-you usually cut them in a spray-& are very free flowering.They are originally bred from hybrid tea & Rosa multiflora.
Grandiflora roses are in-between Hybrid Tea and Floribunda's and have characteristics of both-sometimes they are not separated at all.The term Grandiflora is usually used in the USA to refer to roses like Queen Elizabeth,which is classified in other parts of the world as a Floribunda or a Hybrid Tea-depending on where you live!So the difference is very flexible.
In general,Floribundas have smaller blooms and shorter growth than Grandifloras,whose flowers are more typically 'Hybrid tea' in shape.