Hello Oldsheep
The estring is a silicon ring which is impregnated with oestrogen and releases this constantly at a very low dose. You put it in your vagina as high as it'll go and forget about it for three months, whereupon you need a new one as it will have exhausted the active ingredient. It's a flexible ring which (if I remember rightly -- only seen mine once!) would fit snugly if I brought my thumb and forefinger together in a ring.
It releases 7.5 micrograms daily which is a higher dose than the twice weekly vagifem.
advantages are that it's a low constant dose, so no fluctuations, no other ingredients to irritate, and (if remembering to use vagifem is a problem for you) no need to remember which night to insert your hormone dose.
There is proper scientific info about this somewhere in all the gubbins on the right, and Dancinggirl has just started a thread on it.