The brig is a two-masted, square-rigged ship that has a single gun deck. It is fast, highly maneuverable, and can be used as a merchant ship, war ship, or scouting vessel. Pirates frequently employ them. A brig is arguably different from a brigantine but even sailing enthusiasts might disagree over any differences, and historically the names were interchanged; we can use either term. Early brigantines were often outfitted with oars, allowing for movement with poor winds or in and out of harbor more easily.
The frigate is a fast and highly maneuverable, fourth-rate or fifth-rate ship with one gun deck. These are speedy attack ships and often used as scouting vessels for a fleet, escorting, patrolling or acting independently. They are the largest ships used independently because bigger ships are deemed too valuable to risk being captured or destroyed, which is more likely when sailing alone. Some frigates have two gun decks and are called heavy frigates. The term “frigate” is sometimes used generically to refer to quick ships.
Frigates are the same length as higher rated ships but have fewer decks. They carry supplies for six months. Any areas like a forecastle aren’t as tall as ships-of-the-line. They avoid fights with ships-of-the-line due to being outgunned. During a line of battle, they do not join the firing ships but are instead in another row behind them, relaying signals up and down the battle line, as ships-of-the-line can see only the ship before and behind them, making communication difficult. They also aren’t fired upon by the enemy unless they fire first, as it is considered bad etiquette; they are not only smaller but are present to relay messages, not fight.
Any ship can be turned into a fire ship, but as this means certain destruction to the ship in question, ships that have little to no value are used. The ship is rigged to burn from within the hold, where specially designed tubes vent the flames toward enemy ships and the rigging. The ship is sent floating into the general melee of a sea battle during engagements between fleets; it isn’t something we’d see sailing around by itself. We might assume that something as valuable as a ship-of-the-line is an unlikely fire ship even if damaged beyond any other use; too many men would be required to sail it into position and then abandon it for other ships, but it could be done.