Normal oil filter has all oil flowing through so can only filter to 15 microns in the size available.
Bypass filter is an extra that only allows 10-15% of the oil though at any pass (by having a smaller orifice) so will filter down to 1 micron getting all the crud out of the oil. with good ones oil doesn't even go black! = less engine wear.
Think Discos and rangies have them fitted, but best ones I've come across are www Kleeoil.co.uk