[AshSimmonds]

When you use your phone you will notice that when it's just being a phone, it will have the 3G symbol on the screen but when using it for data you will see H(depends on your phone) that shows it's transferring data.

I got curious about this some time ago as mine is clearly pushing data around even when it just has a G, but then sometimes go all H and stuff.

So yeah, after some searching, a simplistic version of what I heard on the interwebs is that the H means it's doing multiple streams of stuff - so possibly up/downloading simultaneously, whereas G is only one-way data.