A swale is a ditch, most commonly a ditch on contour.
That is also how I think of a
swale.
The soil that is dug
out of the
swale is usually piled on the downhill side, thus creating a
berm.
Quite often, the entire structure (swale and berm) is referred to as a 'swale', which can lead to confusion.
As far as planting trees on a (ripped) keyline goes, I would think that could be a good idea for trees that grow tap roots.