A sewing machine advertised as an upholstery sewing machine is usually a commercial-grade machine. There is such a thing as an upholstery sewing machine, but they vary very little in makeup from an industrial or commercial machine. Upholstery fabrics are difficult to handle on a standard household machines and require a heftier machine than a consumer grade one.
It is possible for some standard home machines to handle heavy fabrics with a change of needle. User's manuals will list the types of fabrics a machine can handle. At its highest capacity, a home machine can take on materials like denim and even leather. Car upholstery and other extreme fabric choices are almost always going to damage a consumer grade machine. Home machines not intended for use with heavy fabrics could end up with problems like broken needles, thread jams, and tension difficulties like bunching.
A commercial or upholstery sewing machine has a few differences from standard machines. The most obvious difference is that they are usually made entirely of metal. In some cases, this makes them difficult to transport. Manufacturers have developed portable versions that break down for easier handling.
The other major difference is the use of a walking foot. Regular machines pull the fabric through by using a feed under the fabric. The walking foot is a two piece foot that alternates between the two halves, so that it looks like it is walking across the fabric. One foot remains on the fabric at all times and keeps fabrics from shifting. This is particularly helpful when working with multilayer projects.
The final large difference between a regular and an upholstery sewing machine is the use of a knee lever. On a standard machine, the only lever to raise the foot is directly behind the needle setup. Commercial machines have a knee activated lever that raises the foot. This makes it easier for users to turn corners and go around awkward places without moving their hands.
Unless otherwise noted, walking foot machines only sew various lengths of straight stitches. If a user requires a different type of stitch, there are machines that specifically do zig-zig and other more complicated stitches. Consumers should check each manufacturer's specifications to see which ones offer the stitches they require.
A commercial or upholstery sewing machine has some options that make it even more useful. There are various feet available to do specific jobs like zippers and welting. A change of needle makes it possible to switch easily between heavy fabrics like corduroy and delicate ones like velvet. With a few small changes, an upholstery sewing machine can serve as many purposes as its user desires.