Random Orbit Sanders
If I was only going to own one handheld power sander, it would be a random orbit sander. Random orbit sanders have a round sanding pad that vibrates (like an orbital sander) while spinning around at the same time.
Random orbit sanders cut more aggressively than orbital sanders but leave fewer sanding marks, even when sanding against the wood grain.
Random orbit sanders can be used both for rough sanding and finish work, depending on the grit of sandpaper and speed of the sander.
Sandpaper: Random orbit sanders require special round self-adhesive or hook-and-loop sanding pads.