Early in the space program, disk shaped orbiters were studied. Reentry would be done with the flat bottom, flight in the air would be done as with the flying saucer airplanes the US tested earlier. See Vought Flying Flapjack.
http://en.wikipedia.org/wiki/Vought_XF5U
The problem was that the disk shape does not hold people very well. It is lacking in usable volume. The solution was to extend the top surface upward, but keep it tapered in enough to not overheat during reentry.
The result was the Mercury, Gemini and Apollo capsules. They are a disk shape on the bottom with a cone shape on the top.
Gemini and Apollo capsules had the center of gravity offset from the center a bit, this gave them some lift, can't remember if the lift was toward or away from the offset. This lift vector gave these capsules some steering capacity. The capsule was rotated so that the lift vector pointed in the desired direction. Usually this was downrange, by doing this the reentry was spread out over a slightly greater time, which lowered the reentry G forces.
Planned future man carrying orbiter capsules use this modified flying saucer plan form. They will be similar to enlarged Apollo capsules.