White chocolate consists of
cocoa butter,
sugar,
vanilla and
milk, passed through the final stages of chocolate making. The residue
cocoa left in the cocoa butter gives it a chocolatey kind of taste, but there are technically no cocoa solids in there. For this reason it is legally not 'chocolate'.
Some brands of "white chocolate" contain vegetable oils in stead of cocoa butter. It is highly debatable if those can still be called
chocolate.
Source: The Chocolate FAQ (www.choco.com/faq.html)