Actually it's more like five categories: fill to as many friends as you have capacity for.
A Friend without Children: To talk to about things besides kids. To tell stories to without being immediately one-upped.
A Friend with Kids the Same Genders and Approximate Ages: To have playdates and commiserate with.
A Friend with Kids the Same Gender, but Two Sizes Bigger: To provide hand-me-downs. Preferably this friend should have generous grandparents and good taste. This is actually a category that almost certainly needs more than one friend, especially if you have boys.
A Friend with Preteens or Teenagers: To hire for yard work and babysitting, and to remind you that it's really not that long before your children will be wiping their own noses.
A Friend with Grown Children: For advice, and to reassure you that your children may still grow up into sane, emotionally-stable adults even though you lock them in their rooms for two hours while you take a nap.