Use a short-pile roller when painting smooth surfaces, and a long-pile roller on textured surfaces.
You should use a short-pile or foam roller for gloss, varnish and other water-based paints, as it will help you avoid air bubbles when rolling your paint. By contrast, you should use a medium-pile roller for emulsion paint. Medium-pile rollers are great all-purpose painting tools, providing a good finish for most paints.
Read our paint brush buying guide to learn more about paint brushes and rollers.