Modelling enamels have an oil binder and spirit based liquid (white spirit or turpentine). I use the term ‘Modellilng Enamel’ on purpose to distinguish them from the normal enamel paints that are glossy paints used to cover kitchen appliances and often hardened in a kiln. Modelling enamals are quite different as they are air-drying and can be either a glossy or matt/flat finish. Modelling enamels are really thinned down oil paints and can generally be mixed successfully with oil paints.
Modelling enamels were the first type of paint to be specifically produced for modellers. They were generally offered for sale in small metal tins and the introduction of colours made to exactly match military aircraft and vehicles was revolutionary and welcomed with open arms by modellers worldwide.
Today, enamels are widely available from many manufacturers in a huge range of colours matched only by the ranges of acrylics available. Like acrylics they cover well and produce a durable finish. They are not as user friendly as acrylics because they have to be thinned with spirits that are inflammable, toxic and smell bad, but this disadvantage should not be over emphasised. Providing the room is well ventilated there should not normally be a problem. Enamels have the advantage that they are slower drying and even after they have become touch dry they can be softened again and removed with spirits which makes them less stressful when used in airbrushes.
I will not express any opinion as to whether acrylics or enamels are best because to favour either one would lead to me deeply offending a large part of the modelling community. Suffice it to say that I use both regularly and would hate to be without either.
Safety Advice: Whatever type of paint you are using you should wear a good respirator when airbrushing.