SPOILER ALERT!!! Scroll down to read my quick summary of the Lost finale. Or not if you don’t want to.

Except for the flash-sideways in the last season it was all real.

Jack and Kate killed the man in black. Jack saved the island. The good guys won. Jack died on the island after saving it.

Hurley went on to be the new protector of the island. Hurley made Ben his right-hand man. They ran the island for an unspecified period of time after the events in the finale. We can assume they found a way to send Desmond (and maybe others remaining on the island) back to the real world.

The airplane gang (Miles, Lapedis, Richard, Sawyer, Claire, and Kate) made it off the island and went on to live the rest of their lives in the real world.

Richard apparently becomes mortal.

The flash-sideways is a kind of purgatory. At some point each character must remember their real life and be reunited with each other. They are then free to “move on”. Presumably to heaven, although the writers were careful to include symbols from several major religions in the scene where Jack is talking with his father. So “moving on” is open to interpretation depending on your religious point of view. The flash-sideways world exists outside of time. Everybody may have died at different times but they were all reunited in this one place. The specific people in the flash-sideways were put together because of their shared experience on the island which was the most important of their lives.

The origins of the island and why it is special are never explained. I’m fine with this. We don’t need it to be explained. Some things need to remain a mystery.