Simply put, they are modified oils. Alkyds are oil paints that are made with an alkyd resin binder modified with a non-yellowing drying oil for optimum color retention, excellent durability, and rapid drying time.
Synthetic oil is used as opposed to a natural binder such as linseed oil.
The painted surface dries to a uniform semi-gloss surface, and all colors dry at the same rate. Artists' paintings done in alkyd oils dry in as little as 12 to 24 hours and can be varnished in as few as 30 days. In contrast, conventional artists' oils can take from three to six months to dry adequately enough for varnish application. Alkyds are applied in one of two methods. These layers generally begin with a base of alkyd gesso, rather than acrylic gesso.