Short answers, pulled from the story.
A metalworking planer is a machine tool that cuts by moving the workpiece across a stationary single-point blade. The workpiece is clamped to a long driven bed and carried past the blade with each stroke, removing metal on each pass. Flat surfaces, keyways, and irregularly shaped internal features are among its typical products.
A metalworking planer is similar to a shaper but larger, and it holds its cutting tool stationary while the workpiece moves. A shaper moves the cutting tool while the workpiece remains still. Both use a linear toolpath, but the planer is suited to larger workpieces.
No single inventor can be credited with the metalworking planer. In the late 1810s, James Fox, George Rennie, Matthew Murray, Joseph Clement, and Richard Roberts each developed the machine independently in separate British workshops. Their work was largely undocumented, and Roe's 1916 account acknowledged the full story would probably never be known.
Metal planers come in two designs: double-housing and open-side. A double-housing planer has vertical supports on both sides of its long bed. An open-side planer has a support on only one side, allowing the workpiece to extend beyond the bed's edge and making it suitable for oversized material.
Metalworking planers are still used by smaller tool and die shops inside larger production facilities to maintain and repair large stamping dies and plastic injection molds. They also handle blocks of metal four feet by eight feet or larger when a horizontal grinder or floor mill is unavailable, too expensive, or impractical.
Metal planers range from a table size of 30 inches by 72 inches to 20 feet by 62 feet. In weight, they range from around 20,000 pounds at the smaller end to more than a million pounds at the largest.