Mops are used in both commercial and residential cleaning. There are different kinds for different purposes and needs. When buying a mop, it is often best to consider the size of floor, the type of floor and the type of job to be done.
Wet mops are usually used on hard flooring for spills and cleaning up. The bigger the head, the bigger the floor surface it will cover. A wringer pail should do the work of squeezing out extra water, and you should not twist the mop in the wringer too much or you will wear it out.
String wet mops are usually cotton or cotton blend and have cut ends or looped ends. Cotton is very absorbent, but may shed lint. Cut ends tend to fray faster and are not usually machine washable as they tend to tangle easily. Looped ended versions tend to cover the floor area more evenly and they reduce the chances of strings tangling. Looped mops are often more expensive than cut versions, but usually last longer as they are machine washable and can be reused many times.
Microfiber wet mops are used for quick cleaning and regular wipe ups throughout the day. They are made of very thin fiber that is good for getting into small spaces. Though many are designed to be used with water, some aren't. Rayon mops do not shed lint as much as cotton ones, and have fast-absorbing qualities. Consider using a mop with a wider band if you need to scrub chunkier pieces of grime off of floors. One with a narrow band that attaches the strands can not usually be used for scrubbing.
Consider buying a sponge mop if you have a small area to clean. This eliminates the need for a wringer, as most have a lever and rollers or a metal plate that squeezes water out of the sponge. Decide which type of wringing action you would prefer using and be sure you are able to find replacement heads for the sponge mop you choose.
A dust mop is used on hard and hardwood floors. These are usually available in many different widths, so you need to consider room size when shopping for one. You can add a treatment to help the head pick up dust, but be sure the treatment product is safe for your floors. Dust mops are usually cotton, but can also be rayon and nylon.
Robotic mops, like iRobot's Scoomba, are also being marketed and are advantageous because they can do the work for you while you spend your time doing other things. They will set you back a bit though, as they can cost several hundred dollars.