It's In the Cards  

Before being spun into thread the wool is often  carded to straighten the
fibers and to help remove bits of grass, dirt and other impurities that may
remain in the wool

Traditionally, wool was carded using cards like those below. A card is a
wooden brush, sort of like a big dog brush, with lots of wire teeth. Cards'
teeth come in different sizes for different types of fibers.

The picture below shows how wool is loaded onto a card.
(The fiber in this picture is a type of hair, not wool)
(Below Left ) This is how the cards are used.  The wool is brushed until
it is smooth.  

Wool can also be sent to companies that have machines for carding,
and small carding machines are also available for home use.
Above Right:  The way that the wool or fiber is carded
determines what the finished thread will look like.

For some fluffy woolen yarn, you would card the wool  
and then roll it into a soft, fluffy roll called a rolag.

For a smoother worsted yarn made of wool, you would
card the fiber smooth.