The difference between an illusion and a trick is that an illusion generally is large-scale and involves a human assistant and/or a large animal. Johnny Carson once asked David Copperfield what the difference was between a magician and an illusionist. David flatly replied, "about $40,000."
The basic definition of a stage illusion is any effect that involves a human being or a large animal like a tiger or elephant. The Pendragons, Lance Burton, The Gamesters, David Copperfield, Sigfried & Roy are among the magicians who can clearly be classified as illusionists.
These grand illusion effects include "Metamorphosis" (an incredible effect involving a locked trunk where the magician and his assistant transpose places within seconds), the "Doll House Illusion" (an illusion where an assistant appears inside of obviously empty doll-house and the "Lady-to-Tiger Illusion" which does exactly as its name implies.
This branch of magic generally requires a great deal of money, an in-depth knowledge of magical principles, at least one assistant, a thorough understanding of stagecraft, staggering talent, imposing stage-presence, prodigious theatrical training, specific venues and great connections.
Many illusions actually require the magician and all of his or her assistants to be in top physical health. It's simply not for the beginner. This is not to say that the intermediate magician couldn't start to include one of two illusions into their act. But the inclusion of an illusion and possibly a trained assistant will raise your costs and therefore your price, making shows at this level fewer and farther between.
