There are three core essentials:
- Chef's knife
- 8" or 10" depending on your preferences
- Paring knife
- 3" or 4" depending on your preferences
- Bread knife
- As long as possible, 12"+
- Feel free to go cheap here, it's serrated and thus largely unsharpenable
You may want to check out Alton Brown's book, Alton Brown's Gear For Your Kitchen. He spends a chapter on knives and where to go past the essentials. He also suggests which ones are worth spending money on and which should be throwaways.