The Island appears to be a tropical island in the Pacific Ocean, approximately 1,000 miles from Fiji. However, it has been said that the Island has the ability to relocate its physical position which defies the natural laws of the planet.
The Island contains a large amount of special electromagnetic energy. People who have arrived on the island, from the ancient civilizations, to the military, to Dharma, have all dug pits to find the pockets of the power. This energy was said to heal sicknesses including cancer, paralysis, brain damage, male infertility, but causes pregnant women to reject embryos which kills the mother and the fetus. However, the healing power does not cure gunshot wounds, poisonous spider bites, strangulation, or spinal tumors.
The Island's electromagnetism may have created a shell (Desmond called it "a snow globe") barrier or shield to hide it from the outside world. The power of the EM fields allows teletransportation of people from the island to the mainland (through the turning of the FDW in an underground chamber). Despite being difficult to find, normal people like Dharma and Ben's people routinely left the island by submarine to bring supplies and new recruits to the island. However, there is a reference that only Jacob, as the island's guardian, had the power to bring people to the island.
According to Jacob, the Island acts as a cork, holding back a malevolent force that would destroy the world if released. When the MIB made contact with the site of this suppression, the light cave, he transformed into a smoke monster that plagued the Island for thousands of years. A guardian protects the Heart of the Island, the source of life, death, and rebirth. Despite the Heart, or possibly because of it, not all who die on the Island move on - some remain, whispering. Other ghostly images of various origin also appear, often confronting people with images from their past.
The concept that the island's heart (a human trait) is the source of all life, death and rebirth may be literal or it may be a metaphor. But those who have been on the island for some time begin to reference it as a person. Some examples of this personification are:
Locke told Jack that the Island brought all the survivors to it.
After his first encounter with the Monster, Locke stated, "I looked into the eye of this Island, and what I saw... was beautiful."
Ben told Locke that the Island is very selective regarding whom it chooses to heal. He added, "I can't wait to show you what this Island can do."
The Island chose not to heal/prevent Ben's tumor and Jack's appendicitis.
Tom told Michael off the Island that the Island wouldn't let him commit suicide because Michael still had "work to do."
Eloise tells Desmond that "the Island isn't done with you yet."
One could argue that the "island" is a mere misnomer for the island's guardian, Jacob, who was the mastermind behind all the events that engulfed the characters on his island.
One further personification of the Island comes from Rousseau's map of the island.
So what is the Island?
Fans have had various diverse theories of what the Island was:
Electromagnetic hidden bubble, like a Bermuda Triangle
Hell
Purgatory
Gateway to spirit world
A malfunctioning spaceship in need of a “pilot”
Island is a quantum dimension where everything is real, but behaves in unpredicted and unintuitive way. The source is a quantum phenomenon, which make time and space to behave in a strange way. The quantum phenomena here means, that each point in time and space is not a definite point, so you can't be sure about anything around the source and everything can become something else.
The deranged mind of a person or a collection of persons (possibly institutionalized or part of a grand experiment on coma patients.)
An interactive, open and unlimited mind connected video game where the players subconscious is the game controller(s)
The Island is whatever anyone wants it to be; a "freedom station" to work out one's deepest problems. The Island, simply put, is whatever you want it to be. It is a place where people can come to start their lives anew (Tabula Rasa) despite whatever sins they've committed in the past. The past cannot be changed (as Daniel Faraday and Miles example) and it is only the future that matters. The Island's healing properties promote fast recovery (various examples including Ben Linus) and promote new life (Ji Yeon's (Jin and Sun's daughter) conception). Other rewards include increased longevity (Richard Alpert) and possibly the reincarnation (John Locke, others) and resurrection (John Locke?, Sayid?).
The Island could be a "wishing well." The Island can do anything requested - it simply requires a sacrifice (or a "toll") as proof that those seeking to be free are ready, willing and devoted to their new path in the world. The character's deepest wishes can come true if they are true to themselves.
The Island was a living, intellectually advanced organism. It brings human souls to it as pets, playthings or experiments to learn about humanity. It is neither good or evil. It is not judgmental or caring. It's power is beyond normal comprehension so it is seen as a god-like being to lesser species.
And finally, a new theory: that the Island was Jacob's sideways world. He constructed it in order to "awaken" his dead brother, MIB, to remember his long lost humanity, so they both could move on together in the after life.
All in all, the Island was a main character of the show. It housed the most mysteries. It still has the most unanswered questions. It is the one character that allowed fans to use their own creative imaginations to synthesize their observations into a credible final premise.
