In reply to LeeWood:
Isn't a corner a style of groove?
I think we'd all think that any 'V' shaped fissure in the rock, regardless of angle of said 'V'(shallow, wide or acute, deep) as being a groove and not limited to 'V' shape either (the grooves up Devils Tower).
I would say a corner is generally accepted as being 90 degrees and I've heard of overhanging corners and slabby ones but a groove can be any sort of recess that is, to a lesser or greater extent, continuous (sentry box etc not being a groove)
In short a groove is what people think is a groove.