Curing agents also known as hardeners, curatives and sometimes catalyst are chemical compounds that react with epoxy resins of different types creating a solid that exhibits high chemical and  physical properties.
A great number of curing agents exist for every different application and can be formulated to achieve specific results in a given resin-hardener system. 
Curing agents vary in viscosity, color, curing times, mixing ratios and they have a big percentage in controlling adhesion, compression, flexibilty, impact resistance, chemical resistance, working times, exothermic properties, thixotropy, electrical resistance, etc.
The possibilities are almost countless when it comes to achieving a succesfull product.