The geodetic effect: the amount by which the Earth warps the local space-time in which it resides (also known as de Sitter precession);
The frame-dragging effect: the amount by which the rotating Earth drags its local space-time around with it (known as Lense-Thirring precession).
In other words, de Sitter is simply the warping effect on space-time of a large near-by mass (The Earth) while Lense-Thirring is an additional warping effect of that mass moving through space.
These two effects could be measured by the amount in which a carefully aligned and highly accurate gyroscope deviated over time. A deviation of the rotating spin axis in the plane of the satellite's polar orbit would reveal the geodetic effect, while a change with reference to the plane of the earth's rotation would reveal the frame-dragging effect.
Project-member and low-temperature physicist William Fairbank observed: "No mission could be simpler than Gravity Probe B. It's just a star, a telescope, and a spinning sphere."
Perhaps a little more than that - the spinning sphere was the most highly accurate sphere every constructed by man (in fact while only one was required for the experiments, 4 were included in the mission for redundancy); and the whole thing was surrounded by 2,440 litres of liquid Helium to maintain a sufficiently low temperature for the year-long experiment.
When considering the accuracy of the gyroscope spheres, NASA notes that if they were the same diameter as the Earth, the difference between the deepest ravine and the tallest mountain would be just 3.6 metres.
Originally planned to be launched via the Space Shuttle, the Challenger disaster in 1986 caused the program to be re-configured for individual launch as a dedicated satellite. This occurred on April 20th 2004.
After 4 months of system checking, the craft commenced collecting science data on August 28th, concluding on August 15th the following year; leaving just sufficient time to run 6 weeks worth of post-experiment calibration before the liquid Helium was exhausted.
During the extended life of the project, the team laid claim to a large number of major technological advances:
At least a dozen new technologies had to be invented and perfected to carry out this experiment. For example, the spherical gyros are over a million times more stable than the best navigational gyros. The pingpong-ball-sized rotors in these gyros had to be so perfectly spherical and homogeneous that it took more than 10 years and a whole new set of manufacturing techniques to produce them. They're now listed in the Guinness Database of Records as the world's roundest man-made objects. The SQUIDs are so sensitive that they can detect a gyro tilt corresponding to 0.1 milliarcsecond. Over its 40+ year life span, spin-offs from GP-B have yielded many technological, commercial, and social benefits - e.g. GP-B's porous plug for controlling helium in space was essential to several other vital NASA missions. Most important, GP-B has had a profound effect on the lives and careers of numerous faculty and students - graduate, undergraduate and high school - including 79 PhD dissertations at Stanford and 13 elsewhere. GP-B alumni include the first U. S. woman astronaut, an aerospace CEO, and a Nobel laureate.
While announcing very close confirmation of the predictions on the NASA site, the full results were said to be available at Physical Review Letters, although at the time of writing, they could not be located.