A preform is a piece of solder (available in a wide variety of shapes). Often, preforms are coated with solder flux or have flux in the center of the metal. Preforms with flux in the center are called flux-cored.
Preforms come in six solder families: gallium, bismuth, indium, tin, lead, and gold-based. There are more than 200 different alloys that can be formed into preforms.