If you've ever looked up into a tree and seen a cluster or ball of twigs, you've seen a witches'-broom. This happens when many small twigs start growing in the same location. This can be caused by several different factors. The tree should be carefully inspected so that the cause of this problem can be determined, if possible, as some are pest- and disease-related.
One cause of a witches'-broom is mistletoe.
This parasitic plant attaches to the branches so it can share the plant's water and nutrients. The witches'-broom will form near these mistletoe-infested branches.
Sometimes these will form because the tree is stressed from a branch that broke off by accident or pruning that was not done properly.
Two other reasons that a tree will develop a witches'-broom is that an insect is attacking the plant or a disease has been introduced.
The witches'-broom itself usually doesn't affect the tree much. You can simply prune it out if you don't care for the way it looks. The tree may need to be treated if the cause behind it (pests and diseases) can be otherwise detrimental to the tree.