Many egrets are members of the genera Egretta or Ardea which contain other species named as herons rather than egrets. The distinction between a heron and an egret is rather vague, and depends more on appearance than biology. The word "egret" comes from the French word "aigrette", referring to the long filamentous feathers that seem to cascade down an egret's back during the breeding season.
They were hunted to extinction in Britain during the 19th Century, mainly because the aforementioned feathers were so sought after.
However, they have been very succesful in recolonising the UK with four species existing here now..