a lot really, the thread forms are different aswell, one has 60 degree inc flanks and the other is a whitworth form of 55 degree inc angles. Bsp has the whitworth form of 55 degree angles.

Bsp is a parrallel thread form where the NPT is a tapered form that runs at 1'47" taper

The NPT is 27 tpi where the Bsp is 28 tpi

Unless you meant to say Bspt which is different again

Hope this helps