The expansion pack to Command & Conquer: Generals was developed and published by Electronic Arts and released in September of 2003. It is rated T.
The Command & Conquer series has a long history of expansion packs. With the release of this one, every game in the series proper has at least one expansion pack. Naturally, some of these are better than others, and more worth the price of entry.
Zero Hour is easily one of the better ones. One of the complaints about this expansion is that some of the additions are so important, so seemingly integral to the game, that they should have been in the game in the first place. These are primarily interface changes (such as putting additional special abilities on the sidebar beyond the normal General promotion abilities), though some of the new units might fall under this heading, too.
The biggest gameplay addition would be the new "specialized" teams. Each of the three base teams (USA, China, and the GLA) has an additional three teams based off of them. These teams are all each specialized in a given area, and are given weaknesses in another area, usually the area most diametrically opposed to the one they are specialized in.
Listing all nine of the new teams would be tedious and boring, so I'll simply mention of few of the more interesting ones:
One of my personal favorites is China's infantry general. Normally, China's greatest strength would be their tanks; they can build many of them, and they gain a "horde" bonus when many of them are together. The infantry general's weakness, interestingly, is an inability to build tanks. They can still build some of the vehicles, such as the artillery and the flame tank, but they don't have the heavy armor.
In exchange, they get the Minigunner infantry unit, and the Assault Transport. The Minigunner replaces the Red Guard unit (China's normal, rifle-wielding infantry unit), and is more expensive than it. The Minigunner has these two advantages over the Red Guard unit: 1) Their gun is fully automatic. 2) They can target aircraft. The gun is also, just for shits and giggles, much more powerful.
The aforementioned Assault Transport is a vehicle any infantry unit can enter and fire out of from the inside, and which also comes with eight Minigunners at no additional cost when you build one. In addition, all of their infantry units start at the first level of veterancy (as opposed to not starting at any level), and they can start all Minigunners at the second level with a General's Point.
Another of my favorites is the US's Air Force general. Somewhat along the lines of China's infantry general, he gets vastly improved aircraft at the cost of not being able to build normal tanks. The primary improvement is the addition of point-defense lasers on many of the aircraft, which make them almost absurdly difficult to hit with anti-aircraft missiles, and they are tougher, besides, if a missile does get through.
Other noteworthies are China's tank general, GLA's camoflage general, and the US's laser general.
The expansion adds new campaigns for each of the three sides, as well. Plot was not at all a strong point of the original game, and this tradition is carried on here. The campaigns play as little more than a series of missions with objectives, with no real overarching goal beyond "smash the enemies!" The missions are solid, however, and entertaining enough.
Naturally, there is a new gameplay mode beyond the new campaigns and the old skirmish mode: the General's Challenge. In this mode, you choose one of the nine new generals and must face off, one by one, against six or seven of the others (for some reason, the infantry and demolitions generals are not included; you will fight seven generals if you play as one of these, and six otherwise). Once you have done that, you must play a final round against the Super General.
The Super General (as I have dubbed her -- yeah, her) is nominally Chinese, but is actually made up of a hodge-pogde of the best units from each of the three sides. The American Paladin tank and Avenger defensive unit, the Chinese hacker and Overlord tank, and the GLA tunnel system are some of her most notable features. (Interestingly, she doesn't get anything more powerful in the air than the Chinese MiG fighter and Helix 'copter.) Beyond all of that, she also has access to all three of the superweapons, which can keep one on their toes.
Generals is one of the best RTSes of the current generation, and this expansion is nothing but good. Highly recommended.