Does it need a compressor?

Yes. It is an air-powered sander, so it runs off compressed air rather than a wall outlet. Match it to a compressor that can sustain the tool's air demand at working pressure.

What pads fit the M14 spindle?

Any 6-inch backing pad with an M14 female thread. That covers most standard hook-and-loop pads, so you are not tied to Eastwood's own pads.

Can it be used for wet sanding?

The tool itself is air driven, but check the backing pad and abrasive you fit. Use a pad rated for water and keep the air motor clear of moisture.

How does it compare with an electric 6-inch orbital?

It is far lighter and has no cord, but it depends on your air supply and lacks built-in dust extraction. Electric units are simpler for occasional use; this one suits a shop with good air.

What speed should I use?

Run slower speeds for finish sanding and higher speeds for stock removal. Because speed follows air flow, set it at the tool and confirm the pad is spinning evenly before you start.