Borewlls in certain areas will have hard water. Hard water means water in which chemicals or salts of sodium, magnesium, calcium, iron etc are present dissolved in the water, much more than the normal water we get from rivers .
When the hard water is heated the water evaporates and the dissolved salts settle down in the vessel. Depending on the quantum of hardness and the kind of chemicals , they form thick or thin crusts and lining in the vessel. You need acidic cleaners to remove them. However by continuous and regular heating f such water the layer of coating becomes thicker and thicker and becomes difficult to remove.
Even without heating the salts can form crust if left untouched for days, as the water gets evaporated slowly by itself. That is what you see in plastic mug.
People who are accustomed to soft water from rivers and Municipal water supply, will not like the hard water. However when the total dissolved salts are very much high, then that can cause harm by long use, like kidney stones, problems for your digestive system etc. Soaps wont lather easily in hard water. Cooking using hard water will result in using more fuel, and sometimes even white rice coked may appear yellow or slight brown.
What is the solution for hard water wit dissolved salts?
RO filters(Reverse Osmosis filters) with TDS control are the best solution for purifying the hard water. Generally the municipal water supply keep the TDS between 35 to 50. So the TDS control may be fixed in your RO filter to give output water having TDS around 50. That will taste normal.
Now you have various brands of RO filters with TDS control. You may choose a suitable one considering the after sales service at your place. ( I was using Kent RO filter when in one place where I had to use hard water. I had a satisfactory experience with that)