Note the microscopic view of wool fibers on the left. Looking at a another microscopic view of the wool fiber where scales can be seen in the photo at Samaurai Knitter at http://samuraiknitter.blogspot.com/2007/04/washing-wool.html
When hot water, soap and agitation is introduced to wool fibers the scales of the fibers open and then tangle together forming wool felt. Other animal hairs can be felted for example the beaver hair is felted and used for hats. Watching the Olympics 2012 Dressage, note the top hats and also at http://www.christys-hats.com/products/fur-dressage-hat for a closer view.
Felt was first invented by a monk, St. Clement who put sheep’s wool in his sandles to relieve blisters. The combination of perspiration from his feet and the motion of walking and pressure from his feet created felt. Before weaving fabric was discovered, felt was the first fabric. St. Clement is the patron saint of hat makers.