Al B's Chipmunk and Squirrel differentiation email.  Thanks Al!  And you're pretty smart about PFO's too!
 

Chipmunks and squirrels differ physically in several ways. A chipmunk can be identified by the five dark brown stripes that run from its head to its tail, which is often upright. A squirrel, meanwhile, has thirteen stripes in total that are also dark brown and run the length of its body. A squirrel does not, however, have stripes on its face. A chipmunk is also usually smaller than a squirrel. Squirrels can weigh between four and eight ounces, while chipmunks often weigh half that. Chipmunks run quickly, with their tails straight in the air. A squirrel runs with its tail at a horizontal angle.

A chipmunk also has pouches inside its cheeks, which it uses for storing food like seeds, nuts and fruit while foraging. Tree squirrels do not have these pouches, though some ground squirrels do. Both animals hibernate, but while a squirrel usually eats more prior to winter to increase its body mass enough to sustain it through the long months, a chipmunk eats food stored in its pouches as well as in and around its burrow during that time.

Chipmunks and squirrels also prefer different habitats. A chipmunk can mostly be found in forested areas, where it can find trees and bushes that provide fruit, nuts and seeds. Ground squirrels, however, like to live in grassy areas, which why they are often seen in parks and golf courses. Chipmunks and squirrels also burrow differently. A squirrel’s burrow can be identified by the mound of dirt at its entrance, while a chipmunk’s burrow is clear of all dirt and is usually located at the base of a tree or bush.