According to Charles Darwin and also the modern synthesis, this distinction is relative and purely a matter of scale. The theory further maintains that species are a statistical rather than ontological phenomena. Neo-Darwinian theory maintains that all changes in gene-frequencies, regardless of scale, are explained by the same observable, natural forces.
In the creationist hypothesis of intelligent design this distinction is absolute and central. Intelligent design advocates argue that microevolution may be explained by constant, observable, natural forces, but that macroevolution must be explained by other forces.
External Resources
Macroevolution as an independent discipline
Macroevolution in the 21st Century
Macroevolution as the common descent of all life
29+ Evidences for Macroevolution