They are quite similar. But the definitions vary depending on the source. Cannelloni translates to big tubes in Italian and Manicotti loosely translates to sleeves. Traditionally, manicotti is made from crepes like in my  Cheese Manicotti Recipe, but I've made them with pasta shells, too. Cannelloni originally was made with pasta sheets that were rolled into tubes, but with the availability of dried manicotti shells, only traditionalists make their own pasta tubes. Most Americans don't know the difference and use these terms interchangeably.