These pots or window boxes have a reservoir at the bottom that is covered with a porous surface. There is a spout that leads out the side of the pot or straight up to the rim. You fill it with soil and add your plants as normal.
Instead of pouring water on the soil, however, you pour it through the spout and fill the reservoir.
The advantage is that you can add a lot more water at once without soaking your plants. You can go longer between waterings which is great if you travel a lot, are busy, or just don't like to deal with watering. It's also great in dryer, warmer climates when plants may need water more often.
The plants soak up only as much waters at they need. This leads to healthier plants that are less subject to root rot. This is important, because I have probably killed more plants by over watering than under watering.
But they are expensive. I find it tough to pay $25-$50 for an ugly, plastic planter just because it is self watering. A standard plastic planter will be under $10. Of course I can go to Wal*mart and find some smaller one for under $10 but I prefer not to do that.
So maybe this year, I'll try making my own. It doesn't look too tough (famous last words, right). I found this video over on VideoJugg.
Gardening Basics:How To Make A Self-Watering Container
For those looking for a hipper version, check out this YouTube video from SurviveLA.com
Are there any other instruction sets for self watering containers you would recommend?