Epoxy or polyepoxide is a thermosetting epoxide polymer that cures when mixed with a catalyzing agent or "hardener". Most common epoxy resins are produced from a reaction between Epichlorohydrin & Bisphenol A. The first commercial attempts to prepare resins from epichlorohydrin occurred in 1927 in the United States. Credit for the first synthesis of Bisphenol A based epoxy resins are shared by Dr. Pierre Castan of Switzerland and Dr. S.O. Greenlee in the United States in 1936. Dr. Castan's work was licensed by Ciba, Ltd. of Switzerland and Ciba went on to become one of the 3 major epoxy resin producers worldwide. (The epoxy business of Ciba was spun-off and later sold in the late 1990's and is now the Advanced Materials Business unit of Huntsman Corporation of the United States.) Dr. Greenlee's work was for a company called Devoe-Reynolds of the United States. Devoe-Reynolds was a player in the early days of the epoxy resin industry, but later sold its business to Shell Chemical (now Resolution Polymers).
Today the epoxy industry amounts to more than $5 billion in North America and about $15 billion world-wide. It is made up of approximately 50 - 100 manufacturers of basic or commodity epoxy resins and hardeners of which the big 3 are Resolution Polymers (formerly Shell; whose epoxy trade name is "Epon"), Dow Chemical (trade name "D.E.R."), & Huntsman Advanced Materials (formerly Ciba; trade name "Araldite"). The other 50+ smaller epoxy manufacturers primarily produce epoxies only regionally (not world-wide), produce epoxy hardeners only, produce specialty epoxies, or produce epoxy modifiers.
These commodity epoxy manufacturers mentioned above typically do not sell epoxy resins in a form usable to most end users, so there is another group of companies that purchase epoxy raw materials from the major producers and then compounds (blends, modifies, or otherwise customizes) epoxy systems out of these raw materials. This class of companies is typically known as "formulators". The vast majority of the epoxy systems sold is produced by these smaller formulators and they account for greater than 60% of the dollar value of the overall epoxy market. There are hundred of ways that these formulators can modify epoxies - by adding mineral fillers (ex. talc, silica, alumina, etc.), by adding flexibilizers, viscosity reducers, colorants, thickeners, accelerators, adhesion promoters, etc. These modifications are made to reduce costs, to improve performance, and to improve processing convenience. As a result a typical formulator sells dozens, hundreds, or even thousands of formulations - each carefully tailored to the requirements of a particular application or market.
The applications for epoxy based materials are extensive and include coatings, adhesives and composite materials like carbon fibre and glass-reinforced plastic (although polyester, vinyl ester, and other thermosetting resins are also used for glass-reinforced plastic). The chemistry of epoxies and the range of commercially available variations allows cure polymers to be produced with a very broad range of properties. In general, epoxies are known for their excellent adhesion, chemical and heat resistance, good to excellent mechanical properties and very good electrical insulating properties, but almost any property can be modified (for example silver-filled epoxies with good electrical conductivity are widely available even though epoxies are typically electrically insulating).
Epoxies find significant use in many applications including the following:
Paints & coatings:
Examples include powder coatings for washers, driers and other "white goods". Epoxy coatings are also widely used as primers to improve the adhesion of automotive and marine paints especially on metal surfaces where corrosion (rusting) resistance is important. Metal cans and containers are often coated with epoxy coatings to prevent rusting especially for foods like tomatoes that are acidic. Epoxy resins are also used for high performance & decorative flooring applications especially terrazzo flooring, Chip Flooring  (http://www.epoxy.com/chip.htm)and coloured aggregate flooring  (http://www.epoxy.com/15.htm).
Epoxy adhesives are a major part of the class of adhesives called "structural adhesives" or "engineering adhesives" (which also includes polyurethane, acrylic, cyanoacrylate, and other chemistries.) These high performance adhesives are used in the construction of airplanes, automobiles, bikes, golf clubs, skis, snow boards, and many other applications where high strength bonds are required. Epoxy adhesives can be developed that meet almost any application. They are exceptional adhesives for wood, metal, glass, stone, and some plastics. They can be made flexible or rigid, transparent or opaque/coloured, fast setting or extremely slow. Epoxy adhesives are almost unmatched in heat and chemical resistance among common adhesives. In general, epoxy adhesives cured with heat will be more heat and chemical resistant then the same formulation cured at room temperature.
Industrial tooling & composites:
Epoxy systems are also used in industrial tooling applications to produce moulds, master models, laminates, castings, fixtures, and other industrial production aids. This "plastic tooling" replaces metal, wood and other traditional materials and generally improved the efficiency and either lowers the overall cost or shortens the lead-time for many industrial processes. Epoxies are also used in producing fibre reinforced or composite parts. They are more expensive than polyester resins and vinyl ester resins, but generally produce stronger more temperature resistant composite parts.
Electrical & Electronics:
Epoxy resin formulations are also important in the electronics industry and are used in many parts of electrical systems. In electrical power generation, epoxy systems encapsulate or coat motors, generators, transformers, switchgear, bushings, and insulators. Epoxy resins are excellent electrical insulation materials and they protect electrical components from short circuiting and from dust, humidity and other environmental factors that could damage the electrical equipment. In the electronics industry, epoxy resins are the primary resin used in making printed circuit boards. The largest volume type of circuit board - an "FR4 board" - is nothing but a sandwich of several layers of glass cloth bonded together into a composite by an epoxy resin. Epoxy resins are also used in the production of circuit traces on the circuit boards and are a major component of the green solder mask used on many circuit boards. Other miscellaneous electronic applications exist in the production and assembly of electronic components.
Consumer & marine applications
Epoxies are sold in many hardware stores - typically as two component kits. They are also sold in many boat shops as repair resins for marine applications. Epoxies typically are not the outer layer of a boat because they are negatively affected by long term exposure to UV light. But they are often used during boat repair and assembly and then are over coated with polyester gel coats or marine varnishes that protect the epoxies from UV exposure. Epoxies are fairly easy to distinguish from polyester thermosets, as commercially marketed epoxy materials typically use 1:1 ratio of resin to hardener, or similar convenient mix ratio, while polyester thermoset materials typically use a ratio of at least 10:1 between resin to hardener (or "catalyst." Also, epoxy materials tend to harden somewhat more gradually, while polyester materials tend to harden more abruptly.