Forward slip = a slip with the intention of losing altitude. Your ground path may or may not be aligned with your nose, depending on the degree of rudder and applicable aileron you use. If your ground path is aligned with your nose, it could also be said that you're in a side slip. For a forward slip power is to be at idle.
Side slip = a slip with the intention of keeping your nose lined up with your path along the ground. This is typically used on landing to touch down straight while in a crosswind. If there is no crosswind, you won't be able to keep your longitudinal axis lined up with your ground path while in a slip. For a side slip while on approach, power is typically used.