A burl is a growth on the trunk and can be found on most species. There are separate opinions on the cause of burls. It can be changes in cellular structures or simply a twig not strong enough to grow through the bark, winding itself to a wooden ball.
The fibers follow the form of the burl which makes it a good choice for bowls and cups.
Burls have thick cell walls which makes them very hard. Burl wood is curly and is especially birch burls are used for knife handles, both for its strength and for its beauty.