WebMD offers a slideshow of common rashes that is useful for identifying rashes on the legs or other parts of the body. A rash may be a minor problem or may signify something more ominous.
Herpes zoster, or shingles, causes a rash that usually appears on the buttocks and trunk but may also appear elsewhere on the body, reports WebMD. The rash features raised dots that eventually turn into blisters. The typical outbreak of shingles lasts for about two weeks, but the itching, numbness and pain caused by shingles can linger for life.
Hives is another type of rash that may appear anywhere on the body, including the legs. Hives causes a welt-like rash, notes WebMD. Sometimes the welts may join together. The condition can last for a few minutes up to days, and results from allergies to foods, medications or additives in foods, or from infections such as strep throat.
Psoriasis causes thick red patches to form on the skin with silvery or white scales, according to WebMD. This rash can occur on the knees, elbows, lower back and elsewhere, and the rash may go away and then return later on.
Rashes that result from contact with poison oak, sumac or ivy cause swelling and redness at the site of contact, notes WebMD. The rash usually forms in a line when a plant drags across the skin. Blisters appear at the site within 12 to 72 hours of contact.