• Microwave: For chopped chocolate, use 50% power (the time depends on the amount). Then stir the chocolate so it melts evenly. Continue microwaving until all the chocolate is melted.
• Stovetop: Place chopped chocolate in a heatproof bowl set over a saucepan filled with 1 inch of barely simmering water. Be sure the bowl is not touching the water or the chocolate could scorch, and stir the chocolate. Do not let any moisture get into the chocolate — not even a tiny drop — or it can "seize" and result in grainy clumps.
Chocolate bars vs. chips
Chocolate chips have less cocoa butter, i.e. less fat, than chocolate bars. This results in them holding their shape better when baked, as well as being cheaper. They have more emulsifiers than a chocolate bar, also a factor in them keeping their shape. Chips work better in some recipes where the distinct shape is preferred or if the end product (frosting, for example) should be creamy and smooth. America's Test Kitchen prefers Ghiradelli 60% Premium Baking Chips as their go-to for dark chocolate chips. For milk chocolate chips, ATK noted that there was minimal difference among brands, but their preference was for Hershey's Kitchens Milk Chocolate Chips, because of its deep flavor and creaminess.
A chocolate fact
The famous Toll House cookie from 1938 used chopped chocolate. The chips weren't created until 1941, prompted by the popularity of that recipe.
From "Everything Chocolate," by America's Test Kitchen