pH
The body spends a lot of its time neutralising acids as most of the food we eat is acid producing. Balancing your pH is a very important function eg. If the blood moves far out of it's range of pH 7.35-7.45 you will die.
Despite the normal homeostatic mechanisms people often report feeling over-loaded with acids. This is not surprising when you realise that the pH of soft drinks like coca cola is around pH 2.5 and eating foods like sugars releases H+ ions (the basis of acidity).
Water (H20)
Water is essential to life: Tap water normally pH's around 7 and contains many dissolved minerals. Water helps remove acid waste from the body via the urine and sweat. Any water or fluid that is acidic will draw down the reserve of alkaline minerals in the body... thus it is not a good idea to drink too many acidic fluids. This also explains why tap/bottled water is normally pH 7 or above.
Calcium
Calcium is one of the key minerals the body uses to balance acids. This loss of calcium to neutralise acids is one possible cause for osteo-porosis as the body uses calcium from the skeleton (including the teeth) to neutralise acids.
Bicarbonate of Soda
A simple way to help boost your bodies alkaline reserves is bicarbonate of soda (NaHCO3) which dissociates into sodium (Na+) and bicarbonate (HCO3-). These are both major ions in the body. You will find both these ions present in most natural mineral waters... and it's a very cheap product at the supermarket NB. Check it's really bicarbonate of soda as sometimes Magnesium phosphate, Sodium Phonsphate and Pottasium Phosphate can be labelled bicarbonate of soda or baking soda... and they're all much more expensive.
Alkaline water
Alkalising machines utilise electricity to electrolyse the water into 2 streams an acid and an alkalkine stream. The alkaline stream contains alkaline minerals and has a negative ORP (Oxygen Reduction Potential) and a pH of greater than 7 (you can set the pH where you like):
Sang Whang gives the most interesting presentation on alkalinity:
Water filters
While some water filters can remove neurotoxins like Fluoride they are often unhelpful because they remove too many minerals N.B. Filters do not change the pH of water. Neither does the ORP of the water change.
Distillation
Distilling water is generally not very useful as this removes all minerals from the water: you get a lot of benefit from the minerals dissolved in the water. Thus water without any minerals can simply draw down your reserve of minerals and can end up making you sick. One way around this problem is to add back minerals after distillation (or filtration).