This new starter has a built in solenoid. To be honest I'm not even sure what a solenoid does.
There are two kinds of solenoids. One is a device that uses a small current to close a high current switch (starter motor, for instance).
Another that is often used on a starter is a combination device that both pushes the starter gear to engage the flywheel and also close the high current switch to the motor to make it spin.
[on edit] Actually there are more than two kinds. There's also the purely electro-mechanical type that just uses electrical current to push a plunger. For instance a high-idle solenoid on a carburetor that holds the throttle plate slightly open.
