What does my serpentine belt run?

Idler Pulleys are generally used to either provide constant tension or make the belt wrap around more of the next pulley, which equals more contact area around that next pulley. An idler pulley, by definition, does not drive a device.

The A/C clutch is used to engage and disengage the A/C compressor so as to provide a more-or-less constant temperature inside the passenger cabin. It's much less complicated than trying to engage or disengage that particular pulley. Another way to look at is that with the A/C clutch disengaged, that pulley is essentially another idler pulley.

An alternator is a type of generator, one that specifically provides AC current. But to describe an alternator as a generator is not inaccurate. To put it another way, an alternator is a sub-type of a generator.

Here's a diagram I found. It may not be exactly the same as your set up, but it should give you the basic idea.