Earth's navy uses many kinds of weapons when outfitting their ships, and they are all listed here. They are:
MAC Gun: The Magnetic Accelerator Cannon is a massive rail gun that runs down the entire length of a ship; resulting in poor fire rate but phenomenal kinetic damage on contact with nearly anything. Fires a 500 metric ton iron-coated tungsten round, maximum effective range of 50km. Ships fitted with these fire at a velocity of mach three for destroyers, all the way up to mach twelve for capital ships and dreadnoughts.
Rail Turret: A considerably smallet turret that uses two electromagnetic, 7m long rails to propell an artillery shell (coated in magnetic metal) at immense speeds.
25-barrel chain gun: A weapon normally reserved for fighter/bombers that fires 3cm long, 5g metal spikes at speeds up to 2 000 rounds per second.
500 Joule laser: A low-powered laser that is meant for close-quarters (ish) ship-ship combat.
1000 Joule laser: A high-powered laser that is meant for ship-ship combat at long range.
Point-defence laser system: A fairly advanced system that uses 250 Joule lasers (30 in a 'set') to destroy oncoming missiles or other large projectiles.
Ion Cannon: An insanely powerful ground-attack weapon that works by using electromagnets to create a 'corridor' of magnetism, that is oon filled with hot plasma, with the magnetic space keeping it from dissapating. The reason it can;t be used against other ships is that the rail guns on most ships will balls with the magnetic fields, causing the plasma to cool too much to do any damage.
Plasma Cannon: Uses the same principals as the ion cannon, but fires massive 'bolts' of plasma (still contained by magnetic fields) instead of a beam. Can be used in ship-ship combat.
75mm: A low-powered cannon meant for ground use or defending against fighter swarms.
125mm: A more powered version used by smaller vessels for ship-ship combat.
150mm: A version featured on larger ships, this one can be used as a bombardment weapon (but it will only have the effect of a 75mm howitzer due to most of the shill burning up in the atmosphere).
200mm: The capitol ship-mounted version that can be used like a 125mm one on the surface of a planet, and also to annihilate frigates and down.
SPARROW system: SPARROW missiles are guided missiles that come in 'pods' of 50 and fire simultanelously at a target. They are usually tipped with 5-metric-tons of explosives.
NOVA Missile: A 4-megaton nuclear missile that usually causes more damae from the EMP blast that comes with detonating a nuke in space than from the plast itself.
PHASE Missile: A 'warp' missile that essentially creates a small, short-lived black hole at the impact point. They can wipe out most smaller ships, but battlecruisers and up appear to be immune (NOTE: If a ship can be powered by creating tiny starts and getting the energy from them, then it's probably possible to create an artificial black hole).