Loud, cramped, smoky and terrifying would probably be a fair way to describe it. Men were assigned to gun crews with specific tasks related to loading the gun, running it out, aiming (in ships that did that), elevating and firing. A normal 32-pound long gun might weigh close to 7.000 pounds, exclusive of its carriage, and would need to be hauled in for loading (or checked at the end of its recoil) then hauled back out for firing. Gun crews would have to stand out of the way because the recoil of the gun would throw it to the end of its breech ropes in less than a second, then snub it again, swab it out, reload and repeat. In well-drilled British ships, guns could manage a broadside every two minutes, and possibly slightly quicker rates of fire individually.

The scene on a gun deck during practice would be extremely loud and chaotic; during a battle, guns could be knocked loose from their mountings and roll around on deck, or explode under rapid fire, causing casualties either way. Cannon balls could enter the ship through gun ports or holes in the hull. A particular hazard was splinters from cannon balls piercing wood (and before anyone brings up the Mythbusters episode, we have ample primary source documentation of the splinter danger from the era). Gun decks were low, between 4' and 6' at the most between one deck and the next deck, so the area to fight in was extremely cramped.

I hope this is a good overview -- let me know if you have other questions. N.A.M. Rodger's The Wooden World is a good overview of this. Here is a brief list of some other books on the topic:

N.A.M. Rodger, "The Safeguard of the Sea."

N.A.M. Rodger, "The Command of the Ocean."

Nathan Miller, "Broadsides: The Age of Fighting Sail."

Neil Hanson, "The Confident Hope of a Miracle."

Ian Toll, "Six Frigates"

Please let me know if you have follow ups. I am not a professional historian of this period but have read about it quite extensively.

Edit: both Safeguard of the Sea and Command of the Ocean have fairly extensive sections on social history, including men and officers.

While probably atypical, and not exactly Napoleonic, one of my favorite stories from the period comes from the Third Battle of Ushant (1794). During the fighting, and the spars of the British Brunswick and the French Veneguer got interlocked. This meant that the lower gun decks were literally pressed together, the cannons almost muzzle-to-muzzle. Both crews fired through closed gun ports just to be able to see what they were aiming at. On the Brunwick, the man handling the worm/wadhook on one of the guns noticed the French crew right across from him loading their gun. Seeing that the French would finish loading before his own crew was able to, he reached his implement into the French ship, snagged the Frenchman holding the round for the cannon, and pulled him out of his ship into the cold water of the English Channel.

EDITED FOR ACCURACY