Quick Question?

Is there actually any difference with Shimano calipers, e.g. if a website is selling a 'front' caliper can it be used on the rear?

Ta

EDIT – sorry I should have said hydraulic disk calipers

If you mean road brakes then Yes the bolt is longer on the front to go through your fork than your frame.

I also think it makes a slight dif with disc's too.

Same calipers, different adapter. A 160mm on the rear would be a 180mm on the front.

The actual calipers themselves are identical (apart from XTR).

PM ones are the same, IS mount ones are different, as nickjb says.

IS rear caliper needs a 160mm disc, if you use it on the front it needs a 180mm disc, if you use an IS front caliper on the rear you need a 140mm disc.

You do just mean the caliper and not complete brake setup where hose length would come in to it?

Yes just the caliper