Mattress firmness is a general term. There is no industry standard for measuring or even defining firmness. At best, it would appear that firmness is a combination of the mattress' support and comfort; however, these are also general terms. Mattress firmness can be described as how much resistance the mattress applies to body weight. Support is related to resistance, but also includes how the mattress sustains and distributes body weight and maintains proper body alignment. The firmness of your mattress is a very important element in the overall effectiveness of your bed. A firm mattress will provide more support for individuals suffering from neck and back problems. Comfort is more difficult to describe. It can loosely refer to the overall "feel" of the mattress. Just because a mattress is firm does not mean it is less comfortable, in fact, many individuals prefer firm mattresses to soft ones.
Mattress firmness is more specifically dictated by the materials, the construction and the depth of the mattress support and mattress comfort layers. The use of coils, foam, air or water in the support layer will determine how much resistance the mattress will apply to body weight. Generally, an innerspring mattress provides the most support and firmness since the coils provide the highest amount of resistance. A mattress with a lower resistance level, and thus less firmness, will result in that soft feeling of "sinking" into bed. The comfort layer provides the immediate impression of firmness or softness, due to a top layer of padding or quilting. Mattresses that are considered softer tend to have a thicker layer of quilting and this additional layer dampens the effect of the coils (or other support materials).
The comfort of the mattress is usually described as being cushion firm, plush, ultra plush, pillow top, ultra plush pillow top and euro-top. Cushion firm provides the least amount of quilting filling or padding and tends to be firmest style of mattress. The amount of filling/padding and level of softness increases with each successive term. The euro-top provides the thickest amount of padding/filling and the highest level of softness.
Ultimately, the level of mattress firmness you choose will depend on your personal preferences as well as your body needs. If you have problems sleeping, ask your doctor to find out which types of mattresses are best suited for you.