Weapon hardpoints will limit the amount of a type of weaponry a ship can use. For example, the Caldari Raven battleship has a total of eight high slots. The ship's bonuses are aimed at missiles so you would normally want to fill all eight slots with missile launchers in order to maximize the damage output. However, the Raven only has six launcher hardpoints which means a maximum of six missile launchers. The other two slots can only be filled with non-launcher modules.
Rig hardpoints allow for the installation of rigs (ship modifications). Rigs are permanently installed on the ship and cannot be removed. If you want to install a new rig in the place of an old one, you must destroy the installed rig first. All ships, with the exception of Tech II ships, have 3 rig hardpoints. Tech II ships have only 2 rig hardpoints. Rigs use a metric called calibration to determine what you can equip. All ships, with the exception of faction and limited edition ships, have a calibration amount of 400 points. Faction and limited edition ships have a calibration amount of 350 points.
Subsystems are modular pieces used to assemble Tech III strategic cruisers. Each strategic cruiser has 5 different subsystems to complete it's assembly. These are: defensive subsystms, offensive subsystems, electronic subsystems, engineering subsystems, and propulsion subsystems. The strategic cruisers require one of each subsystem type to function. The subsystems affect all of the ships attributes and physical appearance.