Felt is a non-constructed textile created from wool or other animal fibres.  Fibres making up the textile are matted together into a non-constructed fabric.  Unlike woven or knitted cloth, felt when cut won't fray.  Whilst it can be made from raw wool in it's natural state, hand made felt is frequently made using "wool tops" or wool batt.  

In the UK "Wool tops" usually refers to the finest wool with the longest fibres.  It's a semi-processed product of raw wool.  Combing and carding removes dirt and short hairs plus aligns the fibres.  What's left - the high quality long fibres - are the wool tops.  Worked well these long fine fibres produce a top quality felt.

Wool batt is fleece wool that has been washed, sorted and then carded. It comes off the carding machinery as a fine web which is collected onto a rotating drum. When taken off the drum this results in a multilayered sheet (batt) of fibres. Because it’s been little processed it often still contains bits of field.

Pre-felt, often known as “needle-punch” in industry, is created through a manufacturing process of dry felting. Machines with beds of thousands of needles mimic the process of hand felting by entangling the fibres to give an even thickness loosely needle felted sheet of fibres. In the commercial world this goes on to be wet felted to produce a hard felt that has applications in many industries. See “What can you do with Felt” if you’re interested in learning a bit more and how the needles in the machines led to the hand needle felting.

Traditional felt is made by adding hot soapy water to wool fibres layered in alternate directions and then applying lots of friction. This method is known as "wet felting".  Like human hair, wool fibres have scales on their outer surface. The hot soapy water used to water down the wool fibres opens these scales.  Agitating/rubbing the fibres together causes the opened scales to interlock. In this way the cloth shrinks as the fibres it is formed from compact, tightening and matting together.  The end product is felt.