Unlike the related Mule Deer (Odocoileus hemionus), which is found in the west and south-west of the US, White-tailed Deer are found through most of North America east of the Rockies Mountains.
The most noticeable differences between White-tailed Deer and Mule Deer are the size and shape of their ears, the color of their tails, and the configuration of their antlers.
Mule Deer have distinct ears set at a 30-degree angle and are larger in size; in fact, this is why they are called "mules". White-tailed Deer have more rounded and erect ears.
The Mule Deer's tail is black-tipped whereas the White-tailed Deer's tail  is not.
White-tailed Deer bucks' antlers consist of a main beam, from which different tines (points) grow upwards. Their antlers are almost always symmetrical.
The Mule Deer buck has antlers forking in two directions as they grow, splitting again to create more tines.
Finally, when running, Mule Deer have a stiff-legged, bounding hop that Whitetails do not have. White-tailed Deer gallop rather than hop.

Top & Left: Pedernales Falls State Park, Johnson City, Texas, USA, 3 November 2006
Middle: Brazos Bend State Park, Texas, USA, 27 October 2006
Right: Big Bend National Park, Texas, USA, 11 April 2007
Last: Fredericksburg, Texas, USA, 2 November 2006