This depends on 2 factors:

1. The internal symmetry of the crystal, and
2. The relative growth rates along the various directions in the of the crystal.

For example, suppose you have mutually perpendicular axes, a, b, and c. Suppose the crystal grows at equal rates along a, b, and c, then the crystal shape will be a cube. Now suppose a different crystal grows fast in the a and b direction, but very slowly in the c direction. The crystal will then grow as thin plates with the face of the plate being perpendicular to c. These are only simple examples.

More complicated cases (and shapes) happen when the crystal doesn't have mutually perpendicular axes, and when the fastest directions of growth correspond to face or body diagonals (or even other directions) in the crystal. More 3D laser crystal could be viewed here

Why crystals grow at different rates in different directions is a very complicated question. If there is a highly attractive interaction (energetically speaking) along a certain direction of a crystal, then that direction will probably grow fast. However, it could also grow slowly, if that direction interacted strongly with the solvent; having strongly absorbed solvent on the surface of the crystal could block growth along that face.