As far as I understand the EGM96 defines the Geoid, where as the WGS84 Standard defines the Ellipsoid.
Is the ellipsoid defined in the WGS84 standard defined in a way to maximize the congruency with the the geoid defined in the EGM96 standard?
a quick google of the two will lead you to http://en.wikipedia.org/wiki/World_Geodetic_System
From that page: Updates and new standards The latest major revision of WGS 84 is also referred to as "Earth Gravitational Model 1996" (EGM96), first published in 1996, with revisions as recent as 2004. This model has the same reference ellipsoid as WGS 84, but has a higher-fidelity geoid (roughly 100 km resolution versus 200 km for the original WGS 84).
Ian's answer is incorrect.
WGS84 approximates Earth by an elipsoid, which is basically a deformed sphere. EGM96 is a more complex model based on the gravitational force of the Earth (which is not constant) that defines what "sea level" or "up/down" mean, a smooth but irregular shape called "geoid". WGS84 is the elipsoid that best fits that geoid, and this fit has been updated as more accurate measurements of the geoid have been carried out over the years. WGS84 is not outdated; it's just a simplified mathematical model used by positioning systems like GPS, even if a geoid is technically more accurate when it comes to define the height over the sea level (since this is different from GPS altitude). You just have to translate coordinates when you need such distinction.