It's a pretty broad collections of polymers that contain a photo-initiator. They are applied to a prepared nail surface and "set" in a lamp, a box with UV light bulbs. Nail gels are used like acrylic nail coatings (with or without extending "tips") to coat the nail and are filled (the growth filled in after two weeks or so) if they are maintained. Gels come in many colors, from clear to polish colors, and are applied clear or clear/pink and white for a natural look or colored for a colored polish look and/or nail art. Some of the earlier nail gels had problems with Allergies but that has been improved for some time.
Nail gels are more flexible than "acrylic" nail coatings (from the same family).
They should be applied with care for the nail and skin around the nail and proper sanitation of tools. Gels are sometime advertised as doing no harm to the natural nail and this isn't true since the surface has to be scuffed, as with any "artificial;" product, so the natural nail has lost it's water proof coating and may peel if the product is removed. If the gel is going to be work for some time this is small issue for the person happy with their nails for many years but should be known if the nails are for a short time or one event.
Here is a great page, explains the chemistry throughly
http://hooked-on-nails.com/chemistry.html
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