A "target" crown makes an attempt at keeping the rifling and bullet in contact with each other for as long as possible - therefore the muzzle is sometimes squared off or or 11 degree - a little more sensitive to bumping around, etc.
Field crowns might have the end of the muzzle rounded over or may have the bore recessed a little (the rifling does not really go all the way to the end of the barrel). This protects the rifling from bumps, scrapes, falls against mud and rocks....sometimes this might be called a "counter-bore".
The target crown is meant to squeeze all the accuracy out of a barrel that is possible, the field crown is meant for hunting, war, etc. It's not that the "field crown" is not accurate, it's just meant for "everyday" use where it might receive a few hard knocks.
