After the skin is cleansed, powder is applied before using soft (a.k.a strip) wax. This helps soak up any excess moisture, and acts as a buffer so that the wax sticks more to the hair than the skin.
Baby powder, which is often talcum powder, can be used. But it also contains fragrance to give it that certain smell we're all familiar with. Fragrance has oil it in, and we really don't want any moisture on the skin. While baby powder isn't going to ruin the job, it's better to sprinkle on pure talcum powder, like Gigi Pre-Epilation Dusting Powder (compare prices).
For those who want to stay away from talc, Shobha makes a talc-free powder to be used with waxing and sugaring.