Alternator
An alternator is a coil that is rotating through a magnetic field and connected to 2 commutators which allow current to pass through the coil. It produces AC (via the Generator Effect) as when the coil rotates the direction of each side changes.
Dynamo
A Dynamo is a coil that is rotating through a magnetic field and connected to a split-ring commutator which allow current to pass through the coil It has 2 different sides with a gap as the coil rotates once it gas past 90 degrees the sides of the coil change which side of the split-ring commutator they are connected to so the side of the coil that is moving down always stays connected to the same side therefore it produces DC (via the Generator Effect) as when the coil rotates the direction of each side changes.