Mucous membranes are located in the mouth, nasal passages, throat, stomach, eyelids, intestines, anus and vulva. They can be thought of as special outer coverings that allow for the easy exchange of nutrients and waste products between the body and the outer environment.
Skin is much less permeable than mucous membranes and chiefly serves as a defensive organ, protecting the flesh and visceral organs underneath from damage due to sunlight and pathogens. Skin does contain pores, which allow for waste excretion, but they are one-way portals that do not allow much to get in. Mucous membranes, in contrast, allow the body to have an interface where things can go in and out. This is because the cells that make up mucous membranes are specialized for the transportation of small particles, such as oxygen and organic molecules.
Mucous membranes are primarily made of epithelial cells. One special kind of epithelial cell is the goblet cell, a cell that secretes a thick liquid called mucus. This sticky fluid serves a variety of purposes depending on the organ in which the tissue is located. For example, in the nasal passages, mucus serves a primarily protective purpose: It keeps dust and germs out of the respiratory passages, while still allowing gas exchange to occur. On the other hand, the thick layers of mucus in the lower digestive tract reduce friction between partially digested food and the inside of the intestines, helping move things along.