Just to be a little more detailed, nitro is nitrocellulose. It's an early-ish plastic finish, and it actually came into pretty wide use in the 20s-30s on cars, the biggest customers for paint finishes at the time. The instrument companies actually started using automotive paint on their guitars starting in the 30s.
The solid resins in nitro are cotton-based, and the solvent evaporates. Unfortunately, the finish melts when it comes into contact with certain rubbers, plastics, and chemicals used to treat leather. So you want to keep it away from the tan surgical rubber and some other rubbers, some plastics (plastics from some guitar straps often leech their solvents onto adjacent materials), and leather, but of course it's ok to use a leather strap with nitro, just take the strap off the guitar when you're done.
Some players love nitro, and so it's still in use, even though acrylic and poly finishes are nominally tougher. PRS V12 is a blend of the nitro stuff and a modern poly finish. PRS also still uses an acrylic on its S2s and SEs, and nitro on a few special runs, Private Stocks, the US made acoustics, etc. The DGT now uses V12.
There's a lot of debate among guitarists as to whether the nitro finish sounds any different than the other finishes. I like the look, feel and tone of guitars finished with nitro, but I'll be the first to admit that there may be a psychological component to the tone part.
Finally, I've found the PRS "shatter hard" nitro to be as tough as most poly finishes in terms of resistance to abrasion and bumps.
