A Hardpoint also called itemport (they are called itemports on some official literature) on a ship is a slot where a certain ship part such as a weapon or another piece of equipment can be mounted. Every ship has multiple different hardpoints which allow for some customization of the ship and its cargo space. There are two major types of hardpoints: internal and external.
As of Star Citizen Alpha 1.1 (March 2015), equipment, component, and hardpoint sizes are based on volume- or the physical space and dimensions that they occupy. All of the items, components, and equipment in the game are set into uniform ranges of volume within arbitrary types of components and items.
The three main pieces of information being (1) Type; (2) Size; (3) Pipes. Type refers to what the item is, such as a Weapon, Computer (Avionics), Power Plant, Thruster, Fuel Tank, Radar, and Shield. Size refers to its volume. Pipes are the ship's internal connections to the hardpoint that power and control the item plugged into the hardpoint.
Essentially it comes down to whether this weapon or component should go into that specific spot. For example, a powerplant is only effective in a powerplant hardpoint. A main thruster is only useful in a main thruster itemport.
Please Note that Classes described on this page and throughout this wiki are obsolete and no longer accurate as of March 2015.
Internal hardpoints hold all ship component systems mounted inside of the ship. Internal hardpoints have a specific maximum volume or size and have a system of pipes that connect them with each other. A hardpoint can only hold a ship part, weapon, equipment if its capacity is large enough for it and where the right combination of pipes are connected to the hardpoint. Small ships generally have smaller hardpoints. Naturally, they cannot equip parts meant for the bigger ships with higher capacity hardpoints.
A ship generally has a hardpoint of a specific type and size with relevant pipes for each of the following ship component systems.
Hull is the ship's bodywork. It holds together the parts of the ship. It also determines its armor rating, mass, and quality.
The power plant (or engine) of a ship is generating power by burning fuel. It is the primary power source for all powered ship component systems. The burned fuel is used as propellant for the thrusters. The power plant is connected to the main power pipe that supplies power and energy to other parts of the ship. It is also connected to a battery- which provides limited back-up power in case of the power plant's failure or destruction.
Avionics is the ship's flight computer system. It calculates the advanced math required for the IFCS which controls the ships fly by wire flight system. It controls the ship's maneuvering and primary thrusters, tracking and identifying targets, and automatically routes power and ensure functional life support. It is unique among systems in that it can also be upgraded with additional systems (e.g. ITTS).
Fuel tank holds the fuel the ship's engine burns. The more fuel your tanks can hold, the farther your ship can fly and the longer it can supply the ship component systems with power.
Afterburners give a significant boost to thrust at the cost of expelling fuel inefficiently. It can aid axial acceleration and increase a ship's maximum forward velocity for a relatively small period of time.
Intakes are the hydrogen ram scoops on the front of the spacecraft which fuel it by gathering the free hydrogen in the space or in the extreme upper atmosphere of gas giants. They refill the Afterburner fuel tank of the ship..
Main thrusters are usually the most powerful and thus the largest ones on a ship. They are the ship's main means of propulsion in the forward direction. The ship's maximum acceleration, range, and radar signature are directly affected by its main thrusters. The capacity of this hardpoint is same as the thrust rating of the thrusters which can be mounted here.
The maneuvering thrusters are used to change the ship’s orientation or to adjust its overall velocity vector in a certain axis. The capacity of this hardpoint is same as the thrust rating of the thrusters which can be mounted here. As of Flight Model 2.0 (October 2015 and Alpha 2.0), the ship's weakest thrust axis directly influences the overall top speed of a ship.
External hardpoints hold equipment mounted on the outside of a ship. External hardpoints function in a similar manner as internal hardpoints. They have a variety of types, sizes (volume ranges), and specific pipes connected to them. Size and type are the most important considerations determining the items that can be equipped or stored at a given hardpoint. A component of a smaller volume can be mounted on a more voluminous (larger) hardpoint, but not the other way around. Primary hardpoints, can attach items of its size range. For example, a Size 3 Fixed Weapon Itemport can equip a Size 3 or smaller Weapon
However, secondary hardpoints/ itemports plugged into a main hardpoint must go down 'one' size. "The rule on sizes for secondary itemports (these are itemports on an item plugged into another item port) is that they have to at least drop a size. So you can only plug a size 1 item into a size 2 item and so on."  Basically a size 3 missile pylon can only mount size 2 missiles. Likewise, a size 2 gimbal weapon mount can only mount a size 1 weapon.
Note that the following information about Classes is out dated. (See Weapons Mount Updates)
Articulated gun (gimbal-mounted). Class 2 hardpoints are rarer and more expensive, but they allow guns to rotate on an arc rather than fire point blank. The increased cone of fire comes at the cost of smaller weapon size.
Turret. Uses the same guns as class 1, but mounted in a separate area of the ship which may be crewed by another person. Turret slots can also mount pylon-type weapons (i.e. a tractor beam or a chaff dispenser.) Some ships can also use these hardpoints for increased cargo space and other ships systems.
Manned point defense turrets, generally mounted on larger ships and capital ships.
Heavy manned capital ship turrets – used for ship-to-ship combat.
Spinal mount lasers - heavy hitters that are effective against medium and large ships.
Capital ship cannons - extended range heavy weapons used for capital ship battles in space.
To be announced. But estimated guess is: Short Range Automated Defensive Systems[source?]
Skin is the paintjob and decals on the hull of a ship. Game-internally it is treated like a hardpoint. Little is known about the Skin hardpoints. The Hornet has two of them, tagged Skin 1 and Skin 2. They have no classification or capacity. We can speculate that Skin 1 makes the basic distinction between the military vs. civilian Hornet skin, and that Skin 2 is used to hold user's custom paintjobs and decals.[source?]
Every ship has a system of internal connections, wires, and pipes that are connected to every hardpoint/ itemport on a ship. These pipes can be damaged and affected by the environment such as by being damaged by a ballistic weapon, energy weapon, or EMP.
There's several major types of pipes which include Power, Data, Heat, and Avionics.