There are two main stances ( Platform and Pinpoint Stance)
Platform Stance
Roger Federer, Novak Djokovic, Pete Sampras, Boris Becker and Tim Henman all use/used the platform stance with slight variations.
In the Platform stance the feet are kept about shoulder-width apart throughout the serve, providing stable support and easy weight transfer from the back to the front foot.
Players will sacrifice some power with the platform serve, but their balance will be better so they should be more consistent. It's still possible to produce good power with the platform serve—Roger Federer uses a variation of a platform stance.
The big advantage of the platform serve is that the player is able to drive up towards contact with the use of both legs.
The other advantage is the variations in the serve's the player can get.
I find that it is easier to teach a player the kick, slice and flat serve from a platform stance rather than the pinpoint.
Roger Federer has probably the best kick (topspin serve) in mens tennis today
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Pete Sampras Platform Stance |
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Roger Federer Platform Stance |
Pinpoint Stance
Maria Sharapova, Rafael Nadal, Andy Murray, Jo Wilfried-Tsonga, Mark Philippoussis, Richard Krajicek and many many more top players use the pinpoint stance.
In the pinpoint stance, the feet start apart, but as the serve unfolds, the back foot slides forward until it is adjacent to the front foot.
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Sharapova Pinpoint Stance |
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Andy Murray Pinpoint Stance |
However if you can hold your balance well, the pinpoint serve provides better velocity, since the feet come together as a single unit, allowing them to push harder against the ground. However, the center of gravity moves more than in the platform stance, so the player must have good body control in order to contact the ball consistently.