Short answers, pulled from the story.
A ship of the line was designed for the line-of-battle tactic, in which two opposing columns of warships maneuvered to fire broadsides at each other. The faction with more cannons firing typically held the advantage, so ships were selected for the line based on the strength of their hulls and the weight of their guns.
The ironclad warship, which emerged starting in 1859, made steam-assisted ships of the line obsolete. The Confederate ironclad demonstrated the decisive vulnerability of wooden hulls on the 8th of March 1862, when it sank two unarmoured Union wooden frigates during the first day of the Battle of Hampton Roads. The ironclad developed over the following decades into the 20th-century battleship.
The 74-gun ship, originally developed by France in the 1730s, offered the best balance of offensive power, cost, and maneuverability. British Surveyor of the Navy Thomas Slade designed new classes of 74s in the 51-to-52-metre range after Britain captured French 74s during the War of Austrian Succession in 1747. Eventually around half of Britain's ships of the line were 74s, and the type remained the favoured design until 1811.
HMS Victory is preserved at Portsmouth as a museum ship and is the only original ship of the line still in existence. She is displayed as she appeared when Admiral Horatio Nelson commanded her at the Battle of Trafalgar in 1805. Although she has been in dry dock since the 1920s, she remains a fully commissioned warship in the Royal Navy and is the oldest commissioned warship in any navy worldwide.
France built the first purpose-built steam battleship, the 90-gun Napoleon, in 1850. Her steam engines could drive her at 12 knots regardless of wind conditions. Eight sister ships were built in France over the following decade, though Britain eventually surpassed France by building 18 new wooden steam battleships and converting 41 older ones.
The heavily armed carrack, first developed in Portugal for trade and war in the Atlantic, was the precursor of the ship of the line. Carracks evolved from a fusion of the North Sea cog and the Mediterranean galley. The galleon later evolved from the carrack during the 16th century and became the main warship type before the formal ship of the line emerged in the 17th century.