Wikipedia is a good place to look for answers to this sort of question. It usually has enough technical information for those who have a science background, but explained in terms which a non scientist has a chance of understanding. Try here - BLICKY(PEDIA).

In a nutshell, Hard Water is alkaline.

In Bradford, the city where I grew up, the main reservoir, Scar House, was built in moorland overlying sandstone. Rain which fell landed on moss and peat, running over the sand and grit stone. The result was extremely soft water.
A friend who was in the motor trade relates once having the tap water tested and proving that it was soft enough to be used for topping up car batteries. He didn't need to buy or produce distilled water for the job.

A few miles down the road, Leeds had reservoirs in a slightly different location. The water in their reservoir catchment landed on moorland overlying limestone. The lime dissolves the calcium carbonate and is hard.

Natural rain is slightly acidic. It contains carbonic acid (from carbon dioide in the atmosphere dissolving in the water).
This will dissolve limestone producing hard water.
Acid rain is usually a result of Sulphuric acid and/or Nitric acid produced as a result of human activity, coal fired power and various industrial processes.
Acid rain dissolves the limestone faster, but is neutralised in the process.

Hope some of that helps.

Quack!
Geoff the Duck.