Food is a big part of a cruise vacation--it seems like every time you turn around there's someone ready to feed you. If you haven't been on a trip yet, you might have the question, "do we need money for food on a cruise?"
The answer is no--sort of.
Your meals (all you can eat) are indeed included, and you can find plenty of afternoon snacks too. What's not included are your drinks. Expect to pay for sodas, alcoholic beverages, and bottled water.
If you've got kids or you're a frequent soda drinker yourself, you can get an unlimited soda pass for about $50 (for a week). (Alas, there's no similar deal for margaritas and mai tais.)
There are also special "alternative" dining options on board most of the big cruise ships. If you want to partake in something besides the buffets and dining room options included in your ticket price (these restaurants are often themed... Mexican, Asian, Italian, etc.), then you'll pay something like $10-$30 per head to eat there. Your regular (free) dining options are perfectly satisfactory however, so you shouldn't feel compelled to eat at these places.
So, the short answer is no, you don't need money for food on a cruise, but you may for drinks depending on your preferences.