Complex sentence: A large sander that is similar in concept to a planer, but is much larger, uses a large sanding belt head instead of a knife cutterhead, and requires air from a separate source to tension the belt.
Split into simpler sentences: A large sander similar in concept to a planer, but much larger. Uses a large sanding belt head instead of a planer's knife cutterhead, and requires air from a separate source to tension the belt.