For example in these images, what are the 'dirty spots' brushes applied to them called?
I am newbie in PS and trying to figure it out.
They look like Paint splatter brushes to me.
Some of them are applied directly to the background directly to the background, using the
Brush Tool (B):
And some of them are being used as masks over other elements (new layer with the brush, then apply as mask over other elements):