I've been blogging about the Six Realms this week. You can't wade very far into Buddhism before bumping into discussion of the Six Realms, so understanding them is useful. But did you know there are 25 other realms?
In old Buddhist cosmology -- which was adapted from very ancient, pre-Buddhist Vedic cosmology -- there were 31 planes of existence, all clustered in layers around great Mount Meru. These 31 planes or realms are sorted into three meta-realms. These were Arupyadhatu, the formless realm; Rupadhatu, the realm of form; and Kamadhatu, the realm of desire.
If you ever run into mention of "Three Realms" or "Three Worlds," usually this is referring to Arupyadhatu, Rupadhatu, and Kamadhatu. And these three realms have a number of sub-realms. "Our" Six Realms are sub-realms of Kamadhatu.
Do you need to know this? Probably not. In case you run into Arupyadhatu and Rupadhatu in your reading, however, just knowing what they are might be useful. I've posted a handy-dandy clip-and-save guide to the 31 realms under the fold --
The three meta-realms plus all the sub and sub-sub realms equal 31. You might run into a dharma wheel with 31 spokes some day, and this is what the 31 spokes represent.
The formless and form realms, Arupyadhatu and Rupadhatu, are populated by devas of various sorts. These devas have transcended the realm of desire, so they aren't the same sort of devas one finds in the Kamadhatu deva realm. When enlightened devas -- gods or goddesses -- pop up in the sutras, I believe they are Rupadhatu devas and not Kamadhatu devas. Being formless, Arupyadhatu devas probably don't make appearances anywhere. But I could be wrong about that.
Other than that -- I think understanding the Six Realms (of Kamadhatu) is helpful for most of us, but I'm not sure how much time anyone needs to spend on the rest of it. It strikes me as a lot of brain clutter, frankly.