Let's blow through this.
Command & Conquer: Generals - Shockwave is a modification for Command & Conquer: Generals - Zero Hour. Shockwave adds a number of new units and abilities to each of the nine Zero Hour generals and their normal 'vanilla' factions.
Many of the additional units are based upon ideas that were clearly intended to be included in Zero Hour's original release. Numerous unit voices, sounds and art references to these missing units can be found within the game's BIG files. Barring very few exceptions, nearly all of the missing upgrades, units and buildings have been introduced in some form within the Shockwave modification.
Among these include the Laser Comanche for Townes; the Hypersonic Aurora for Granger; the Advanced Particle Cannon for Alexander; Fusion Reactor and Weapons-Grade Uranium shells for Tao's tanks; the Demo Battle Bus for Juhziz and the toxin Scorpion for Thrax.
In addition to the extra units, Zero Hour originally intended to present the player with three (not merely one) boss generals in the Generals' Challenge, and include missions featuring General Fai and General Juhziz (which were missing from the final release). Playing the Generals' Challenge mode in unmodified versions of the game will eventually see the player stand off against a Chinese General named Leang, who commands the technology of all three factions at the end.
However, the boss Generals Ironside (of the USA) and Mohmar 'Deathstrike' (of the GLA) were eliminated before the game's release, although their voiceovers and portraits are contained in the game files (but lacking a challenge map).
In addition, General Fai's (of China) challenge mission was also removed. However, General Juhziz (of the GLA) still had a partially complete challenge map.
All three of these boss-generals have been modified and included in Shockwave, utilising the tactics and abilities for which they were supposedly originally intended to command. The Generals' Challenge mission featuring General Juhziz was completed and added back into the game (but General Fai's mission was not).
Shockwave is a partial conversion and thus aims to not change the feel and backstory of the original game. It is comparable to most expansion packs for RTS games in its initial setup, but without patches to the hard-coded executable.
SWR productions staff have stated that they wish for the Shockwave to be thought of as what Zero Hour may have been, had Electronic Arts not cut short the development time.