What is the difference between a wall and a trim paintbrush?

Wall paintbrushes hold more paint and have thicker bristles and trim paintbrushes hold less.

The main difference between wall and trim brushes is the amount of paint they hold. A wall brush like the Purdy Pro-Extra has thicker bristles and will hold more paint because it is designed for longer brush strokes. Trim brushes are designed for shorter brush strokes so hold less paint and offer more control.