electromagnetism - What's the difference between magnetic fields H and B?

As per Wikipedia:
"The term (Magnetic Field) is used for two distinct but closely related fields denoted by the symbols B and H, where H is measured in units of amperes per meter in the SI. B is measured in teslas in the SI."

So, the two are closely related. Why do we need two, then? Could just one be used?

As I remember from the university, for vacuum the Maxwell's equations are written usually in terms of B, while for media in terms of H (and B=uH).