A mop is for "wet cleaning" floors -- for soaping, water washing or rinsing and sopping up the excess dampness. Some mops have sponges at the end of a stick, others have cloths, yarn or fibers. Some mops have a wringer attached to help wring out excess water. In general, mops are designed for use with water and soap, but there is also "dry mopping" with mops designed to pick up dust and dirt without the liquid. Some newer brands of mops are designed with disposable cleaning parts.
Sweep or vacuum before mopping. Mopping large amounts of dirt will just spread it around. Get rid of the majority of the loose dirt and dust first. Then mop.
- about.com on How to Mop a Floor