A rash is a change of the human skin which affects its color, appearance, or texture. A rash may be localized in one part of the body, or affect all the skin. Rashes may cause the skin to change color, itch, become warm, bumpy, chapped, dry, cracked or blistered, swell, and may be painful. The causes, and therefore treatments for rashes, vary widely. Diagnosis must take into account such things as the appearance of the rash, other symptoms, what the patient may have been exposed to, occupation, and occurrence in family members. The diagnosis may confirm any number of conditions. The presence of a rash may aid diagnosis; associated signs and symptoms are diagnostic of certain diseases. For example, the rash in measles is an erythematous, morbilliform, maculopapular rash that begins a few days after the fever starts. It classically starts at the head, and spreads downwards. Differential diagnosis
Common causes of rashes include:

Food allergy
Medication side effects
Anxiety
Allergies, for example to food, dyes, medicines, insect stings, metals such as zinc or nickel; such rashes are often called hives. Skin contact with an irritant
Fungal infection, such as ringworm
Balsam of Peru
Reaction to vaccination
Skin diseases such as eczema or acne
Exposure to sun (sunburn) or heat
Friction due to chafing of the skin
Irritation such as caused by abrasives impregnated in clothing rubbing the skin.
Treatment
Treatment differs according to which rash a patient has been diagnosed with. Common rashes can be easily remedied using steroid topical creams (such as hydrocortisone) or non-steroidal treatments. Many of the medications are available over the counter in the United States.The problem with steroid topical creams i.e. hydrocortisone; is their inability to penetrate the skin through absorption and therefore not be effective in clearing up the affected area, thus rendering the hydrocortisone almost completely ineffective in all except the most mild of cases. References
External links

Guide to rashes on Medline Plus Medical Encyclopedia – includes photographs
Links to pictures of skin rashes (Hardin MD/Univ of Iowa)
Pictures of common skin rashes compared (Dermapics)