The term "ship of the line" is a throwback to Old Earth naval traditions where massive battleships with heavy firepower would form a battle line to engage enemy armadas. For the Terran Protectorate Stellar Navy, the term refers to any capital vessel designed to be the primary long-range weapons platform for a fleet or task force. These ships are meant to be the "big guns" of any fleet engagement, and the ships, along with the carriers, that the rest of the fleet must defend to the death.
Ships of the line include battleships, battlecruisers, dreadnoughts and battlecarriers (battleship/carrier hybrids). They are usually the largest ships in the fleet (with the exception of fleet carriers), carry devastating arsenals of weaponry, often serve as fleet command ships and are heavily armored and surprisingly fast.
The above image shows the four types of Ships of the Line serving in the Terran Protectorate Stellar Navy during its second war with the Imperial Consortium.
The Powell-Class battleship (top) is the oldest ship of the line in service and is also the most common, comprising 70% of the fleet's ships of the line. It's main gun, a gauss cannon that fires a two-ton tungsten slug with an anti-matter core that explodes after it has pierced an enemy ship's hull, is still the largest caliber single weapon of any TPSN starship. It can also fire large spreads of missiles in addition to other weapon systems. The Powell can also carry four fighters. Originally these were fighter/bombers but in later years they were replaced with electronic warfare fighters like the Hawker, which added to the ship's combat effectiveness.
The Hastings-Class Battlecarrier (middle rear) saw its first deployment right at the start of the war and was an attempt to develop a "kitchen sink" ship that does a bit of everything. In addition to being a battleship blistering with weapons, it also carries three squadrons of fighter/bombers and two squadrons of interceptors, for a total of 60 fighter craft, in addition to a number of auxiliary ships. While most ships of the line are only deployed during warfare, the Hastings battlecarriers are slated for use as exploratory and diplomatic vessels once (and if) the war ever ends. Only a handful have been constructed, as they were meant as "testbed" ships for a number of advanced weapons and sensory equipment. Its most noteworthy weapons are its two anti-matter cannons and 24 antiship missile tubes. It carries its own anti-matter production facility, giving it theoretically unlimited range if food, water and air could somehow be replenished.
The Damocles-Class Battlecruiser (middle front) was developed as a result of successful tests of systems used on the Hastings and was the first of a "new generation" of warships deployed once the war was in full swing. It is faster than any of the other ships of the line and while its firepower is impressive, it rates slightly under the armament of the Powell, which earned it the battlecruiser classification instead of a battleship. It also carries a squadron of 12 fighters, typically space superiority fighters like the Crossbow or Tengu and several other auxiliary ships.
The Dreadnought-Class Battlecarrier (bottom) is the largest, most heavily armed ship of the line to date. It carries a pair of powerful gauss cannons, a pair of anti-matter cannons, eight particle beam cannons and 12 anti-ship missile tubes as its primary weapons. There are few ships in known space that could survive a full volley. Unfortunately, most of the ships that could survive are fielded by the enemy. Meant as a siege weapon, the Dreadnought is often-armed with "citybuster" style neutron missiles capable of traversing re-entry and destroying planet-bound enemy assets or space stations. It carries 84 fighters and bombers and, like the Hastings, can generate its own anti-matter for fuel and some of its weapon systems.