High slots are a category of module slot found on ships in EVE. Generally, high slots contain the weapon systems of a ship, but there are plenty of other types of modules that use high slots, from mining lasers to drone upgrades.
Mid slots are a category of module slot found on ships in EVE. Generally, mid slots contain activate-able modules that may assist in tackling, propulsion, or generally improve the ships behavior. Shield tanking modules occupy mid slots. Some mid slot modules share purposes with low slots, but the details of their use differ.
Low slots are a category of module slot found on ships in EVE. Generally, low slots contain passive modules which improve ship performance. Armor tanking modules occupy low slots. Some low slot modules share purposes with mid slot modules, but the details of their use differ.
Rigs are permanent ship modifications that fit into the rigging slots of a ship. Once fit, they must be destroyed in order to be removed (unlike regular modules). Most rigs have some other drawback associated with them; these penalties are reduced by training more levels in the associated rigging skill.
Subsystems are special modules that are used to build T3 ships.