The Rubber Manufacturers Association (RMA) sets the guidelines for determining which tires can be properly designated mud and snow tires. The actual marking you'll see on your tire if it does meet the RMA's standards could be any of the following: MS, M+S, M/S or M&S.
In order for a tire to get the RMA's mud and snow designation it must meet the following geometric design criteria: Tires must have multiple pockets or slots in at least one of its treads that extend toward the tread center at least 1/2 inch (about one centimeter) from the footprint edge, measured perpendicularly to the tread centerline. The tire must also have a minimum cross-sectional width of 1/16 inch, and its edge of pockets or slots has to be at an angle of between 35 and 90 degrees from the direction of travel. In addition, the tire tread contact surface void area should be at least 25 percent, based on mold dimensions.
Basically, what these dimensions require are rows of fairly large grooves that start at the edge of the tire and move toward the center, and that at least 25% of the tire's surface area must be grooves. A tire with enough void space can better grab through the snow or mud to gain traction.
You could think that having an RMA's mud and snow rating meant you were good to go. Unfortunately, as you see from the above, these ratings are based on a tire's geometry -- not how well it does on tests actually driving in mud or snow. The RMA has tried to address this gap by creating a separate designation, called "Severe Snow Use." A tire with this designation has a picture of a little mountain with a snowflake inside of it next to the MS designation. To earn the Severe Snow Use designation, a tire must exceed certain tire traction results as tested in accordance with the American Society for Testing and Materials severe snow test procedures.