A burr is a wart-like growth on the trunk of a tree, which I understand is the result of a virus infection. As in the equivalent human condition, the tissue grows in random fashion, so there are no annual growth rings and the wood is a jumble of whorls in variegated colours. Here's a big burr on the stump of an ancient oak tree.
And if you think that's big, just take a look at this 8-foot-long chunk of elm trunk waiting to be sawn into slabs - it's virtually all highly desirable burrwood!
A slab of burr yew.
