Dermatitis & Eczema
Dermatitis is a catch-all term for inflammation of the skin. Eczema is the same thing – the terms are interchangeable (though some people use the term 'eczema' for specific types of dermatitis, such as atopic dermatitis in children).
And though there are different causes of dermatitis, the effect is usually the same: when the skin is inflamed it gets red, swollen or blistered, and intensely itchy.

Contact Dermatitis is inflammation of the skin due to contact with a particular substance, either an irritating chemical or an allergen (an allergen is something that causes an allergic reaction).
So-called 'irritant' contact dermatitis is caused by the direct chemical effect of substances such as detergents, solvents, acids, alkalis, oils and skin cleansers. It's common among caterers, cleaners, nurses, builders, hairdressers and mechanics. The hands are often affected.
'Allergic' contact dermatitis occurs when the inflammation isn't caused directly by a chemical, but by an allergic reaction in the skin. Common triggers are creams, gels, lotions, and shampoos; antiseptics in hand creams; fragrances and deodorants; jewellery; hair dyes and bleach. Some people are allergic to metals like nickel and cobalt, and get dermatitis from the backing of watches, jean studs or buckles.

Atopic Dermatitis is common in babies and young children. They get a pinkish-red, scaly rash (which is very itchy) on their faces, scalp, behind the ears, over the body and on the arms and legs. The rash can get lumpy, weepy and crusty.
As kids get older, atopic dermatitis tends to clear from the face and is found instead mostly in the elbows, behind the knees, and/or on wrists and hands. If the child scratches the affected area, which they often do because it's so itchy, the skin may become infected.
Atopic dermatitis is more common in children who have other allergies, like asthma and hay fever. It tends to run in families, and it may be triggered by allergens in the environment such as house dust mites, animal dander, grass pollens and moulds. It can flare up and then settle down for no apparent reason.
It usually clears up as the child grows older and is usually gone by adulthood.
Atopic dermatitis can't be cured, but it can be managed.

Seborrheic Dermatitis is a red, itchy, scaly rash on the scalp, face and other areas of the body like the eyebrows, beard, under the breasts, and in the folds of skin in the genital area (these are all areas where there are high numbers of oil glands in the skin).
In adults it affects people aged 20 to 40, men more often than women. Dandruff on the scalp is a mild form of it. In infants when it occurs on the scalp it's known as cradle cap.

Varicose Dermatitis is a blue/brown discolouration of the skin caused by poor venous circulation, usually because of varicose veins. It's most common in the lower legs and ankles, and is also known as stasis dermatitis.
At first, the skin becomes itchy, reddened and mildly scaly. Over several weeks or months, the skin turns dark brown (cased by staining from blood that's seeped out of veins). The skin may break down and form a painful open sore (ulcer), typically near the ankle. It improves with wearing of compression stockings and treatment of the varicose veins.

Nummular Dermatitis is also known as discoid eczema, this condition usually affects middle-aged people. Small round or oval patches with tiny blisters, scabs, and scales can affect any part of the body, especially the lower leg. They may be itchy but not always. The normal unaffected skin may be dry.