The World Health Organisation recommends that babies should be exclusively breast fed for the first six months of life. The full findings of this study can be found on the World Health Organisation website.
For women who make an informed decision not to breastfeed, who feel they need to supplement breast feeding, or are unable to breast feed, there are now huge selections of baby formula which are very close to breast milk.
Stage One formula is suitable for baby from birth and is boasted by formula companies to be almost identical to breast milk, though obviously without the anti-bodies found in breast milk. These are cow's milk based and iron fortified. They are recommended for use up to 6 months old. Stage One formula often also comes in a “hungrier baby” formulation. This is usually the same as the normal Stage One mix but is casein rather than whey dominant so takes longer to digest. Stage One Hungrier Baby formulations can help to delay weaning if a baby is still too young for solids.
If a child is eating a balanced, nutrient rich diet of solids then the use of Stage One formula should be continued however the majority of babies take a little while to adjust to solids and get a rounded diet so Stage Two formula is there to help.
By the time babies reach 6 months they have used up the reserve of iron which they brought from their mother when they were born. As babies grow they also need more calcium. Stage Two formula has extra iron, calcium, proteins and vitamins. It is casein dominant as opposed to the Stage One whey dominant formulas and so takes longer to digest. Stage Two formulas can often be used up until age 2.
An excellent reference for any more specific formula or breastfeeding concerns is Baby Center.