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)
Milk chocolate differs from dark chocolate in sweetness and milk content. The added milk comes in the form of either milk powder, liquid milk or condensed milk. In the US, milk chocolate must have a chocolate liquor concentration of at least 10 percent.
I rarely ever use milk chocolate because it is much more limited than dark chocolate. There is less cocoa butter and solids in milk chocolate and there is much more sugar. This means that there is less control over the sweetness of the final product and there is less chocolate flavor.