The GPS can give you a position and velocity information in regards to a reference model. There are several reference models:
There are differences between each model, and changing from one model to the next one can result in a difference of several tenth meter in the worst case.
The Geoid is an approximation of the Mean Sea Level (MSL). "The MSL is usually described as a tidal datum that is the arithmetic mean of hourly water elevations observed over a specific 19-year cycle. This definition averages effects of the gravitational forces from the moon and sun" . The differences between the Geoid and the MSL are reduced to its minimum on the coastlines, however on some spots there can be a difference of several meters.
The Ellipsoid model uses an easier mathematic construction to fit Earth shape with an ellipsoid. Several Ellipsoid models are available depending on the region of interest. The most well known model is the WGS84 as it its now used by all GPS receivers. An ellipsoid model is supposed to be a close approximation of the MSL, but it appears there are differences of several tenth meters between an ellipsoid and Geoid models.
 - "Mean Sea Level, GPS, and the Geoid", By Witold Fraczek, ESRI Applications Prototype Lab, UrcUser July-September 2003.