From WSU Clark County Extension:
Quaking Aspen is best known as a widely grown native tree with fluttering leaves which seem to “quake” with the slightest breeze.
 
This is broadleaf deciduous tree which grows 40’-50’ tall and attains a width of 20’-30’ at maturity. Leaves are arranged in alternate fashion and have a heart-shaped to nearly round shape. They are typically 1”-3” across and are typically thin in cross section. The leaf margins are finely serrated. As for color, they are dark green on the upper surface and lighter below. In the fall the leaves turn a very pleasing yellow color before they are shed. Leaves are borne on slender, reddish brown twigs.

This is a dioecious species with male and female trees, though some trees will have both male and female flowers. Flowers consist of hanging catkins which are 1”-3” long.

Aspen bark is smooth and creamy yellowish-white to light green when trees are younger. Mature trees will develop thick furrows and can become darker, especially near the base.
 
Quaking Aspen typically grows in full sun though it can tolerate light shade. It is adaptable to nearly all soil types, and can survive and even thrive on wet sites. It is considered a fast growing species, though it is short lived. Aspens spread aggressively through inter-connected root systems.
 
Primary photo credit: James St. John on Flickr (CC BY 2.0 Deed)
Secondary photo credit: Paul Asman and Jill Lenoble on Flickr (CC BY 2.0 Deed)