There are three main families: dark (fondant) chocolate, milk chocolate and white chocolate.
Dark chocolate contains cocoa powder (a dark brown substance), cocoa butter (an oily substance with a broken white colour) and sugar. It may also contain soya lecithin, a natural emulsifier (to unify the chocolate and improve its homogeneity).
Powdered milk is added to obtain milk chocolate.
White chocolate doesn't contain any cocoa powder. It contains only cocoa butter, which leads some to claim that it is not genuine chocolate, but there is no agreement on the issue.