Chocolate is created by a long process that culminates in the grinding and milling of cocoa beans (or nibs) to produce cocoa solids and cocoa butter. The cocoa butter and cocoa solids are them mixed with milk, sugar, and other flavoring components to get the desired taste. The definition of chocolate caries based on the government and country (with lobbying groups constantly trying to reshape these definitions).
White chocolate is technically not chocolate because it does not contain any cocoa solids. However, it does contain an ample amount of cocoa butter giving it the mouthfeel of traditional chocolates. Because caffeine and theobromine are found in the cocoa solids, white chocolate contains only trace amounts of these stimulants.
White chocolate can be substituted in most recipes that call for other forms of chocolate. However, the flavors will certainly be much different. In addition, the melting point of chocolate decreases with the decrease in cocoa content. White chocolate thus has a lower melting point that dark and milk chocolate (about 87F). In addition, white chocolate has a lower burning point (110F).