An ash commonly refers to trees of the genus Fraxinus (from Latin "ash tree") in the olive family Oleaceae. The ashes are usually medium to large trees, mostly deciduous though a few subtropical species are evergreen. The leaves are opposite (rarely in whorls of three), and mostly pinnately-compound, simple in a few species. The seeds, popularly known as keys or helicopter seeds, are a type of fruit known as a samara. The genus Fraxinus contains 45-65 species world-wide.
Unfortunately, both green and white ash populations are being decimated by the emerald ash borer (EAB). Discovered in 2002 near Detroit, MI, the beetle has spread through much of the northern ash range and threatens billions of trees.
The Common North American Ash Species:
Ash has shield-shaped leaf scars (at the point where the leaf breaks away from the twig). The tree has tall, pointed buds above the leaf scars. There are no stipules on ash so no stipulate scars. The tree in winter has pitchfork-like looking limb tips and there could be long and narrow clustered winged seed or samaras. Ash has continuous bundle scars inside leaf scar looks like "smiley face".
Important: A leaf scar is the major identifier when keying a green or white ash. The white ash will have a U shaped leaf scar with the bud inside the dip; the green ash will have a D shaped leaf scar with the bud sitting atop the scar.