Originally Posted by peterj
Distilled water is fine for extract brewing but it is not good for all grain brewing unless you are adding brewing salts (e.g. calcium chloride, gypsum, etc.) to get your ions and mash pH to the correct level. Extract should already have the minerals that the manufacturer used in their mash. Tap water will have some (maybe a lot depending on your water supply) minerals in it already.
Agreed. If you are mashing, you need to consider the relative alkalinity of your mash water and your resultant mash pH. If you get above/below the 5.2-5.6 range, you can get some additional flavors from the mash that are not the best for your beer.
If you are using tap water and know your water report, you can find what kind of beers would naturally work well with your water. However, most homebrewers like to brew a variety of beers, so getting familiar with water chemistry is a good 'next step'.
An evening of searching around will quickly get you some spreadsheets which will guide you.