Those are wasp galls. There are wasp larva growing in there. Most likely, some type of beneficial wasp that preys on pests. Have no fear. As to the type of tree, I should know that, but it resembles some type of ash. I think I'm off the mark on that, though.

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Dave
Even my growlights are getting restless!

Lynne...some insects "host" on only one or only certain species. As to why they're so numerous, most insect species have peak years, or even cycles when they don't appear for years at a time. A good example is the 17 year locust. They spend 17 years underground, then come out for one season.

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Dave
Even my growlights are getting restless!

You wouldn't see an insect until it had begun to feed and get larger. Most of these gall wasps are only a 16th of an inch long when they mature. Keep an eye on those galls, I'll bet they get hollowed out. The wasps feed on the tissue inside once the tiny eggs hatch. The gall provides food, and a safe environment. Then, when the wasp emerges, it makes a tiny hole in the gall and flies away. Most viral galls are irregular.
Let me know what happens, you've got me really curious.

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Dave
Even my growlights are getting restless!