The island is an island. However this island is special. At the core of this island is a force of amazing power, both destructive and good. How this power came to be is not important. Like the force, Gods in religions etc etc this power has always been. It simply IS.
That is what the island is, now for some of the mysterious stuff on it.
At the core/heart/center of this island is some sort of cave/area. This seems to be as close to the sourse of the islands power as we can get. There are carvings and other man made things down here and we can see a pool has been created at the epicenter. Who made this? This is not important, what is important is that this place was made. The show presents us over six seasons with evidence of various people/groups/civilisations coming to the island and trying to harness or understand the islands power. It is clear/likely that this is the earliest group of people to have found the island.
So why did these original inhabitants create "the cork". This is unclear but again it is likely they (and they were likely egyption or some other advanced civilisation) did so with out a clear idea of what would happen. However this is likely to have been when a protector first came about. The release of the power caused by the hole and pool created may have caused some chaos/accident and possibly even transfered power to an inidividual, the first jacob. It may also have created an early Man in Black/Smoke monster, but this is open to interpretation. (I dont think a Man in Black was created prior to the one we met, but some have enough evidence to believe the mother was one too, its fine and doesnt really matter). Anyway, these people, after witnessing what happened needed to plug the whole and hence: The cork.
And now a process of people coming to the island and adding their own geography to it continues for thousands of years. Some people come and take the power to be a god so build the statue in its honour. Similar reasons are taken for the building of the temple. Various protectors live and die during this period and the original goal becomes muddled and unclear. What is left is a strong desire to protect the island.
So across the sea (a terrible episode). We learn that at some point mother becomes the new protector and that her adopted children are Jacob and the MiB.
We learn that the man in black's people are trying to find the source so are digging holes/wells to find it. They decide that by putting a wheel into the source of various power 'hot spots' they can leave the island, but they are not entirely sure how this works.
We can take a stab at it though. Water runs through the heart of the island and appears significant to various other important areas of the island. It is likely that turning the wheel not only releases enough energy to teleport someone off island, but also can change the direction of the waterflow, and hence, change the direction of time.
Other curious things: the healing pool: again water that runs from the source likely runs to the pool. The power of the source is enough to heal people, but there are some differences to this (otherwise jack would have survived). The reason the pool turns dirty is likely because as with the original inhabitants bonding the power or atleast connecting the power of the island to a protector, the water's power is too.
Following these thoughts we can theorise dharma's role and other people who have come to the island.