Broadly there are two types of Mattress Protectors, Waterproof Mattress Protectors and Quilted Mattress Protectors.  Waterproof Mattress Protectors as the name suggests do not let any spillages to seep through to the mattress and the top terry layer absorbs the spillage over a larger area. Additionally, it protects the mattress from dust, dust-mites, etc  Quilted Mattress Protectors while not being waterproof are resistant to spillages and depending on the type of spillage it will protect the mattress from stains. While a thick liquid takes time to seep through while a more highly thin liquid like water penetrates easily.