Many people seem to refer to any highly figured wood or interesting grain pattern as a burl (also referred to as a burr outside the US), but I'm told "true" burls are extremely rare. What is a burl, and what causes it?
A burl is an outgrowth on a tree:
They are cause by infestations or injuries.
Inside the burl the grain is twisted and interlocked, this makes it interesting for carvings and for its strength as it's less likely to split. This grain pattern also makes it difficult to work with.