Answer: Based on your description I suspect it is some sort of gall. These are not uncommon on maples and although they may look odd, usually do not harm the tree. Here is additional information about leaf galls on maples, including a photo. You may need to cut and paste the complete url into your browser to make it work correctly.
If this is not what your tree has, you might consult with your local county extension for a more specific diagnosis. If it is something that requires a chemical control, they will have the most up to date recommendations on what to use and how/when is best to apply it.
Enjoy your gardening!
Q&A Library Searching Tips