As this is a question I hear over and over from parents of new babies and young infants, I thought it was time to try and help clarify why so many exist.  But before moving forward let me give my usual disclaimer...this one being I want everyone to know first and foremost I am a huge believer and promoter of breastfeeding and breast milk.  My children were breastfed and I will always do everything I can to support my mothers aiming to breastfeed as I consider breast milk to be the "gold standard" of early infant nutrition.  However, on occasion, whether it be a personal choice or a decision ultimately reached after exhausting all available resources, sometimes breastfeeding isn't meant to be...and no one should ever second guess that decision because the formulas we have today are pretty darn good.

Also, for those who may be looking for specific brand names, you won't find them here.  I want to be clear within each group of formulas I don't promote one brand over another...so if any questions on specific brands and where they may fall in this list, please feel free to leave a comment and I'll clarify any questions about specific brands in that manner.  So beginning with the most commonly used formulas for full-term infants, I leave you with the 4 main groups:

1. Cow Milk Protein-Based Formulas:  It is from this group of formula we typically begin.  Using breast milk as the gold standard, its composition includes protein, fat, carbohydrates, iron and a variety of other nutrients.  Included in this group of other nutrients, some may have prebiotics and/or probiotics...some of the hyped up nutrients we hear more about these days.  Early studies indicate there may be positive health outcomes from using these nutrients but more studies still need to be done before formal recommendations can be made.  And in regard to iron...as many parents believe it is often the problem of constipation in their babies...it absolutely needs to be included in infant formulas to avoid iron-deficiency anemia.  This means, don't grab the low-iron formulas still to be found on the shelves.
2. Soy Formulas:  In years past, often the formula turned to when it appeared an infant wasn't tolerating the standard cow milk protein-based formula.  Symptoms of a formula intolerance include increased gassiness, irritability/fussiness, increased spitting up and looser mucousy stools often with some blood noted as well.  But with the recent hype and concern surrounding the impact phytoestrogens (found in soy formula) has on a child's endocrine (AKA hormone) system, I typically bypass this formula if a cow milk protein allergy exists.  In fact, there is around a 50% incidence of soy protein allergy if a known cow milk protein allergy already exists.  And as far as the American Academy of Pediatrics (AAP) stance on phytoestrogens...no conclusive evidence exists.  But these days, the AAP still states soy formula should only be used for parents who are strict vegans, if a lactose intolerance is felt to exist (which typically isn't seen until about a year of life), or is a baby has galactosemia (a congenital disorder where a baby cannot digest the carbohydrate galactose). 
3. Protein Hydrolysate Formulas:  These formulas are for those infants with a severe milk or soy protein intolerance and for those with severe malabsorption diseases (including cystic fibrosis, biliary atresia).  The proteins are so hydrolyzed (i.e. broken down) that its components should not cause any allergic/intolerance response.  Two concerns about these formulas include the poor taste and high cost.
4. Amino Acid-Based Formulas:  We turn to these when the protein hydrolysate formulas don't do the trick.  Very expensive, as well.

So turning back to the question of why so many formulas as there are only 4 formula groups for full-term infants...well, because it can be a huge money maker for the manufacturers, lots of competition exists.  In the cow milk protein-based formula group, I'm thinking of 10 different brand names (some being a store brand name) just sitting here. 

Questions?

Dr. Jeremy