The all-important first six weeks in a cat's life will do much in determining his personality and character for the rest of his life. Healthwise, this period is also extremely important to the developing kitten, as very young kittens are susceptible to a number of threats, such as fleas and URIs.
Kittens will probably never grow again at the remarkable rate they accomplish during this period, and seeing the changes in their development from week to week is an incredible experience. We'll start off by recapping the first week, and move on from there.