Autumn Olive
Elaeagnus umbellata (GoBotany, Wikipedia, USDA) is an invasive shrub (usually four to eight feet high, although it can grow into a small 16ft. tall tree) found in Acton. It has a distinctive leaf (dark on top with a light, fuzzy underside), and has large amounts of small red berries (flecked with silver or gold) which ripen in the fall. These are a favorite with birds (and hikers), and result in its spread. The berries are edible, and have large amounts of lycopene in them. They are tart like a tomato, but ten times stronger.
Removal
Autumn Olive is fairly easy to remove.
The plant is very easy to uproot completely. Anything less than a couple of inches in diameter can probably be pulled up. Cutting the plant (the wood is very soft) will result in regrowth, but after a couple of cuttings it will die.
Common Mis-Identifications
Black Cherry (Prunus serotina, Wikipedia, USDA - Factsheet) has a similar leaf. The Autumn Olive bark is much smoother, and the Cherry leaves are serrated and aren't light and fuzzy underneath.
Russian Olive (Elaeagnus angustifolia, Wikipedia, USDA) is another invasive plant with similar leaves. It can grow much larger, and has spines.