Dreadnoughts are, in essence, battleships on steroids. They're the second-largest non-capital ship, and the first most heavily-armed non-capital ship (unless you count the guns on fighters in a carrier's weapon count, in which case the dreadnought is left in the dust).
An Earth Dreadnought is 2,250 metres long, 300 wide, and has four main decks.
The bottom deck is fairly stubby, and is actually a sort of protrusion on the back of the ship from the belly. It contains the ship's 10-gigawatt reactor, and 4 175mm howitzers on each side. The second has ten railguns on each side, the main PHASE engines, the side thrusters, and the dual MAC cannons; as well as the storage space. The third deck has 16 broadside railguns on each side, two back-thrusting, forward-located engines, five forward-facing 125mm howitzers, and four backwards-facing ones; along with the crew quarters and med bay. The fourth is the bridge and server room, and sticks out of of the back-top of the craft. It has 14 PHASE missiles on board, launched from the missile tubes on the third deck.
The ship has four dropship bays, and comes standard with four dropships. It has a crew of 750, with no room for extras.
It carries an AI, with the somewhat-prerequisite GIGA-level mainframe. It is plated in an entire 8m of layered (from the outside in) Composite, vanadium-titatium alloy, vanadium-steel alloy and solid steel.
Dreadnoughts are commonly used as capital ships when proper capital ships aren't present for whatever reason. Note that armament will vary depending on fleet.