Magnets are made of ferromagnetic metals such as iron, steel and nickel. All magnets have a magnetic field, which means that they also attract ferrous materials. The magnetic field is produced by the motion of electrical charges, and causes objects to be either attracted to or repelled by the magnet. Some magnets are temporary, and lose their magnetic filed over time, while others are permanent.
Magnets have poles that are attracted in opposite directions. A typical polar magnet has one end that will aim toward the north if allowed to rest on its own, and an end that will point to the south. The northern facing end is the magnet's North Pole, and the southern facing end is the magnet's South Pole. If you have two magnets, the North Poles will repel each other, but be attracted to the South Poles. Likewise, the South Poles will repel each other while being attracted to the North Poles.
Water magnet simply indicates a magnet that is used to treat water somehow. Some people believe that using magnets on water will alter the water's feel and healthfulness. Generally, water magnets are used on water that is often reheated or meant for a pool. It is thought that by either fixing magnets outside of or within pipes, they will prevent buildup in the water and soften it by removing hard elements attracted to the magnets.
There is no real difference between polar magnets and the magnets used for water treatment, other than that they are used for different purposes. Water magnets are simply polar magnets placed on or within water pipes. While magnets are widely used on refrigerators and lockers to hold items to the surface, the effectiveness of magnets on water is not accepted by the majority of the mainstream.