What is the difference between whole milk and homogenized milk, if there is one?

Milk is homogenized by pumping it through a restricted orifice at high pressure, which fragments the fat globules and reduces the fat globule size to avoid separation or forming a cream layer at the top of the bottle or carton of fluid milk. Thus, homogenization creates a homogeneous distribution of milk fat throughout the container of milk. Homogenized milk can be purchased as whole milk (which must contain 3.25% fat), reduced fat (2%), low fat (1%), and no fat or skim milk (0-0.5% fat).

Mike Hutjens, Extension Dairy Specialist, University of Illinois – Illini DairyNET

Additional reading:
Stuart Patton. Milk. Transaction Publs., New Brunswick, NJ. 2005.