Allergic contact dermatitis is a delayed type hypersensitivity (type 4). Also here. Click on links for photos and more information.
Allergic contact dermatitis is often called eczema. This is a descriptive word that is applied to a wide variety of skin diseases. The most common type of eczema is atopic dermatitis. It is an allergic condition that makes your skin dry and itchy. It is most common in babies and children.
Irritant contact dermatitis is caused by direct contact of certain chemicals with skin. No allergy is involved. Examples would be diaper rash or "dishpan hands".