While some use the terms interchangeably, mattress pads and mattress toppers may mean different things to some manufacturers. A pad is a thin, quilted material that fits atop the mattress, offering a little extra protection from moisture or stains. A topper usually refers to thicker add-ons that also rest atop the mattresses. Toppers add extra firmness or softness and may be used to make the bed more comfortable.
While some companies use mattress pad as a catch-all term for both pads and toppers, much of the time a pad is the thinner of the two. A typical mattress pad is thin and quilted, made of washable fabric with batting inside, providing a thin layer of extra comfort for you and a little protection for the mattress.
The pad may attach to the mattress with elastic straps, or it may be designed like a fitted sheet, except with a meshlike material on the sides. The mattress pad protects the top of the mattress from general wear and tear. A fitted sheet fits over the mattress pad as if the pad isn't there.
A mattress topper is usually thicker than a mattress pad, designed to offer extra firmness or softness to make sleep more comfortable. Thick mattress pads change the feel of the mattresses beneath them.
Latex or visco-elastic foam adds firmness to the feel of the mattress. Down or down-alternative toppers make a firm mattress feel softer.
The thickness of the topper often determines the level of comfort offered; the thicker the pad, the more noticeable the difference in the feel of the bed. Any topper less than 2 inches thick won't offer much support.