Skin Shielding: Skin shielding is a form-fitting shield that forms a "sheath" over the object in question. Mark I is the least powerful, and used by more infantry, whereas Mark X is the highest power and is used on coreships for protection.

PPC: Plasma Projector Cannon - compresses plasma into a coherent beam or "shell" using gravitons and magnetic fields.

MAC: Magnetically Accelerated Cartridge - a gun that uses magnets instead of an explosion to accelerate the shell.

U-x: Ultra, weapon sized to destroy planets and/or capital ships.

H-x: Heavy, larger high-yield version of the weapon, usually low recycle rate and lots of waste heat and energy demands.

M-x/No prefix: Medium, normal version of the weapon, average recycle rate, overall balanced.

L-x: Light version of a particular weapon, usually do less damage overall but have a rapid recycle rate, and produce little waste heat and don't require too much to power.

x-X: This suffix denotes that the MAC cannon uses special shells filled with explosives of some sort.

Coreship: a massive battleship or carrier (5 - 15 km, usually) that forms the "core" of the fleet. Usually obscenely hard to destroy, it acts as a command ship for the rest of the fleet and the personal ship of an admiral or high-ranking official.

Capital Ship: Capital ships are ships usually 100m - 5km. They form the next largest layer of the fleet, and are usually the ships in the thick of the fighting.

Battleship - larger, heavily armed and armored ship that is difficult to destroy, hard to defend against, but is maintenance intensive and expensive, slow, and cumbersome..
Battlecruiser - large capital ship that trades firepower for armor. Slight advantage over battleships in speed and maneuverability. Credit to The Mindset for this definition.
Cruiser - medium or large capital ship of middle-range firepower and armor. Slightly easier to maintain, and much less expensive.
Light Cruiser - medium sized capital ship with many middle-range weapons. Highly vulnerable to anti-capital ship weapons, but better able to evade them.
Destroyer - Small to medium sized capital ship designed for a tactical role; i.e. minesweeper, cloak-detector, blockade runner. Usually cannot hold their own in a straight fight versus a larger vessel, but are far more maneuverable.

Smaller Craft: This includes corvettes and fighters, though fighters are usually not counted as actual ships in their own right.

Corvettes: smaller, lightly armored, armed, and maneuverable craft, corvettes offer a cheap and affective defense versus fightercraft and bombers, and when armed correctly can even punch out capital ships.
Bombers: while not counted as actual vessels in their own right, dedicated bombers are essential to any fleet for the threat they pose to capital ships
Fighter-Bomber: heavy multi-role craft, fighter bombers can usually be used to throw the knockout punch at a capital ship, then rejoin battle against fighter craft once the heavy payloads have been spent.
Fighters: dedicated fighters usually have but one purpose: to find and kill bombers and other fighters. They are usually lightly armed with fast-repeating weapons. A veteran fighter squadron can prove a deadly force to any ship, however.