Question

Whats the difference between B field and H field?

Answer 1

The H field is that field produced by the, usually electric current, source. It's something like H is proportional to I, with all sorts of turns and things in the calculation. That's the SOURCE of the magnetic effect.
The B field is measuring how the SOURCE affects different substances, with a vacuum included for a reason I don't understand. So for a given current and stuff - turns, etc - there's one H field, but, if that H field comes across, say, a paramagnetic substance, then that substance will become magnetised by induction and there'll be an associated B field with it. If the H field comes across, say, a ferro magnetic substance, then that substance will become magnetised - possibly a lot more so - with a different B field and so on for all magnetically susceptible substances. I the case of the ferro, if the H field induces magnetisation, and the H field then is removed, the ferro remains magnetised, and this is called REMANENCE.
This may help though you may find it waffly
http://perendis.webs.com

Answer 2

for a solenoid - n is turns per unit length
H=nI
\[B=\mu _{0}(H+M)\]
M is magnetisation which depends on the substance - ie susceptibility