+1 to all the above. I'll just add...
First fix (aka framing nailer - American??)
2.8mm thick nails from 50mm - 90mm, usually paper-collated. Construction use, stud walls ('framing') etc...
Second fix (aka 'finish' nailer)
16g (1.2mm) 32 - 63mm long, stripnails, can be straight or angled. Used for e.g. skirtings and architraves i.e. where the nails are visible and the 'finish' is important.
Pin gun (aka brad gun or bradder)
18g (1mm) 15-50mm long, stripnails. Mouldings and general purpose pinning e.g. while glue sets, quadrant or scotia edging around floors etc...
Also 21g (0.8mm) 'veneer pinner' and 23g (0.6mm) 'headless pinner' 15-30mm long for very delicate work, fine mouldings etc...
Stapler - most likely narrow-crown stapler, typically 20-25mm long, great for fixing thinner materials where a pin may pull through e.g. a 6mm back to a cabinet.
That's my basic take on it, my uses. HTH
Peter