Use dynamic_cast when casting from a base class type to a derived class type. It checks that the object being cast is actually of the derived class type and returns a null pointer if the object is not of the desired type (unless you're casting to a reference type -- then it throws a bad_cast exception).
Use static_cast if this extra check is not necessary. since dynamic_cast performs the extra check, it requires RTTI information and thus has a greater runtime overhead, whereas static_cast is performed at compile-time.
