Hard water is the name given to water which contains magnesium and calcium minerals. Hard water becomes such when rainwater, which is naturally soft, falls on porous ground, such as limestone or chalk. As the rainwater penetrates the porous ground, it picks up the magnesium and calcium particles along the way, which it retains once it gets to the reservoir and when it enters your home.
  
There are many effects of hard water, the majority of which are detrimental to your home. Due to the mineral content of hard water, it creates limescale build up in your home which makes it appear unclean and clogs up shower heads and even corrodes the inside of your appliances, reducing their efficiency and lifespan.
The composition of hard water means that it is also damaging to the natural layer that protects your skin, and some studies have shown that there is a correlation between hard water and dry skin conditions like eczema.