Cotton is by far one of the most absorbent textiles ever produced. It can hold many times its weight in water. In fact, it is used in the fabrication of not only clothes, but other products that are used to absorb liquid, more so than any other material. It is the fibers that make up cotton that causes it to absorb water. Natural fabrics tend to be much more absorbent and more comfortable than synthetic fabrics. The looseness of the fibers of the natural fabric also allow for it to be more breathable, making it a better choice for your skin.
Cotton is much more absorbent than many other fabrics because of its molecular structure. The arrangement of the molecules in cotton creates an abundance of places for water to be attracted to. Other fabrics, especially synthetic ones, have fewer places for water molecules to bond to, therefore even though other fabrics will attract and hold some water, they do not have the capacity to absorb like cotton fibers.
Cotton is better than almost all fabrics at absorbency, although other natural fabrics like linen and wool are also absorbent. Nylon, polyester and acrylic are poor in terms of absorbency. They do not absorb moisture and are uncomfortable to wear when you get wet. Acetate, rayon and the newest fabric, lyocell, which is referred to as a rayon subcategory, all have good absorbency. Some of the literature on lyocell claims that it is even more absorbent than cotton.
If you are looking for an item of clothing that will absorb sweat from your body allowing you to stay cool and comfortable while exercising or in hot weather, cotton is a good choice. Some synthetic fabrics are likely to dry quickly and as thus are touted as good choices for layers while exercising. However, these clothes do not let your skin breathe properly and you run the risk of fungal infections and the like. If you are looking for softness, absorbency and the ability to breathe in your fabric of choice, natural cotton is definitely the way to go.