All other ingredients are called adjuncts and are used to compensate for where the beer's flavor falls short or to enhance the overall experience. Here is some very basic information provided by Chico's Home Brew Shop.
Yeast does all the work of making alcohol and carbon dioxide out of malt sugars.
Experimentation is key in order to experience the differences between them. Pure strain liquid cultures are better than dry yeast because they are free of bacteria and give a consistent and predictable result. The basic rule to tap into about water is that it is alright to brew with it if it is all right to drink it. It is always better to be safe. Letner's advice is to use sterile water, so it is best to boil.