Cotton is a strong fiber, but has a fairly short staple, or fiber length. When treated to be "mercerized", cottons are stronger, smoother and less prone to shrinkage than untreated cottons. The yarn label will indicate whether the cotton is mercerized or not.
Cotton is often blended with wool, making it softer and warmer. When blended with linen, it becomes stronger and makes the linen softer.
Cottons are great for all kinds of projects, from afghans to baby items to sweaters. Cotton is also frequently blended with wool and nylon for socks, resulting in a really comfortable sock.
Cotton fiber is also susceptible to attack by moths, so use mothballs to protect your yarn and garments. If you don't like the odor, periodically place the yarn or garment in a plastic bag in the freezer for 48 hours to kill moth larvae. You should also air out stored fibers in the sun every now and then.