"Homeworld: Cataclysm was originally developed in 2000 as an expansion of Homeworld, but was released as a stand-alone game. It was produced by Sierra Entertainment, as was the original, but it was developed by Barking Dog Studios.
Though it uses the same engine as its predecessor, several changes were made such as: the ability to toggle time compression between normal speed and eight times faster; ship upgrades (improving armor and adding new abilities), Command Ships and Carriers are given the ability to add external modules for ship research and fleet support; fuel was completely eliminated from the game and finally the sensor display could be used to issue attack orders to units. The player's Command Ship is now capable of attack; though slow, the Command Ship is capable of delivering a vast array of weaponry, most notably the Siege Cannon capable of severely crippling an enemy Command Ship in a single shot if fired accurately.
Notable unit changes include the Processor, Cataclysm's adaption of the Resource Controller, which has medium-strength weapons to defend itself; automated repair beams to heal near by ships and four pads to dock with Workers harvesting resources. The game's resource collectors perform the same functions that they did in the original Homeworld, however, when upgraded they can be used to capture enemy vessels, harvest crystals and repair friendly vessels; functions that were carried out by separate, single-function ships in the first game.
The game also introduced new 3D features such as moving parts and transforming ships.
In general, the main difference is the scale of fleets. Where Homeworld was biased towards large fleets (as the player's main ship was a fully-fledged mothership and the opposition was an empire of galactic scale), Cataclysm down-scales the fleets (as the player's main ship is a simple mining vessel and the adversaries are all limited in resources)
Source: Wikipedia, "Homeworld:_Cataclysm," available under the CC-BY-SA License.