Just to be thorough, I'm including the link to the EarthBox website:
There is a lot of information on that website.
Up above I linked to some planter boxes made by Gardener's Supply, and in her first message ncyg46 linked to a similar product (Garden Patch Grow Box). All of these work by the same principle--a water reservoir inside the box keeps the plant evenly moist. A special soil mix is required to make this work right, which can be purchased or mixed yourself if you have the ingredients.
A good book on the topic of self-watering container gardening is by Edward C. Smith, "Incredible Vegetables from Self-Watering Containers."
I have tried some homemade versions of this type of container and found that they work GREAT. In Nebraska's windy summer heat I was never able to keep things alive in containers--with the self-watering containers they do beautifully. I've grown flowers, eggplant, strawberries, and this year a pepper plant in containers.
I think a number of people on this board grow things in containers, so perhaps there are more people with experience to share about these?