Why "egregores"?
An egregore is almost, but perhaps not quite, a living Being. It is not quite a living being in the full sense because it probably does not have sufficient independence to be called that. Yet it is alive, aware and intelligent (after a fashion), and very active. An egregore comes into existence through the interactions of mortal human beings and divine Beings. This is where the dependency of the egregore comes in. An egregore must be sustained by the ongoing interactions of humans and Spirits, or else the egregore will dissipate and cease to exist.
An egregore is more "real", stable, and powerful than your average "group mind" astral thingy. But it is "lower" than a God-form. God-forms can (thank the Gods) survive for long periods without much (but probably not without any) active participation of human beings. A God-form is not a God, but rather, as the name implies, the "form" of a God that is knowable by humans.
I have read more precise Qabalistic definitions of egregores, especially in the writings of Denning and Phillips (Ogdoadic Magi extraordinaire). Bascically when a sufficient number of people over sufficiently long periods of time share a common set of symbols, ritual acts, etc, with respect to some specific God or group of Gods, a new Being is created: an egregore. The Gods being worshipped already existed, but these Gods have responded to some particular group of humans who have called upon Them. So an egregore is basically a relationship, one might even get all Biblical and say that an egregore is a covenant. But it is not just that - it is a covenant "come to life". And like all that is born, it is mortal.
I like to think of an egregore as a dance in which one partner is human and the other divine. The music is all of the rituals, iconography, etc, that defines a particular "tradition". When the humans and divinities come together, they strike up the music ... and the dance begins.
Gabriel Emerson has put together a compilation of definitions of "egregore":
http://www.chaosmatrix.org/library/chaos/texts/gegregor.html
And here is an essay written by occultist Walter Ernest Butler of the Servants of the Light Mystery School:
http://www.servantsofthelight.org/knowledge/butler-egregore.html
And here is an essay by Phil Hine:
http://www.philhine.org.uk/writings/ess_egregore.html
My interest in the word comes from the fact that an egregore, to the limited extent that I understand the term, only comes into existence when a given spiritual path is lived. It is not a result of mere "belief", it can only come about as a result of what Gandhi-ji called "a constant heart-churn". When Gandhi said that he was talking specifically about renunciation, but really I think he was describing devotion in the truest, widest, and best sense. One common perspective found in Hinduism is that there are (at least) four kinds of Yoga: bhakti (devotion), jnana (knowledge), raja (meditation), and karma (works). I think this is a useful idea - but only if it realized that (1) we must have all four, and (2) bhakti takes precedence over the other three.
Of course the greatest of care is called for in deciding what to devote oneself to.
[The cool image at the top of this post is the logo for a small publishing house called Egregore Press, with which I have no affiliation. But they do have a catchy name, don't they?]