One of these boats was the Black Rock, which brought Richard Alpert. Man in Black knew he could not kill Jacob, but thought that maybe Richard, a slave on the Black Rock, could do it for him. He convinced Richard that he could be with his wife if Richard killed Jacob, and he gave Richard the dagger he'd used to kill Mother.
Jacob overpowered Richard and took the dagger. He explained to Richard a little about the island and what he was trying to do. Richard pointed out that Jacob needed to help guide the people and that he couldn't just expect them to do the things he was hoping they would. Jacob made Richard his adviser and granted him the "gift" of never aging.
Many people came to the island, including a group of hippies that started The Dharma Initiative to study the unique qualities of the electromagnetism of the island. Because of the H-bomb detonating in 1977 (by Juliet) and the resulting radiation, babies conceived on the island would die, along with their mothers, right around the second trimester.
Jacob or Man in Black made it so that people who died on the island, and were not good people, were trapped, hence the whispers.
Finally, on September 22, 2004, 815 crashed and the story of our Losties began. Their time on the island was the most profound part of each of their lives. This included great danger, time travel, and death of people around them.
Eventually, Jack, and then Hurley and Ben, took over as the island's protectors. Ben pointed out to Hurley that Hurley was the protector and he did not have to follow Jacob's rules. He could make his own rules.
One of the rules Hurley made was that after death, the Losties would all find each other and meet in a church, where they would then move on together.
Flashbacks and Flash Forwards - Why the Heck Were These Included?
The flashbacks and flash forwards were added to give more depth to the Losties' stories. They were meant to show us, the viewer, what happened to our characters before and after the present day to give us a better understanding of who they were and the struggles they dealt with.
The Flash-Sideways - What the Heck Were These?
The flash-sideways should be looked at as a separate story. The island story and flashbacks and flash forwards are what happened to our Losties while they were alive. Because while they were on the island was the most significant time in their lives, and because Hurley was the leader of the island and could make his own rules, Hurley made it so that they would all find each other in the sideways after they each died. They would connect to each other, which would awaken their memories, which would lead them to each other, ultimately meeting at the church to move on to whatever is next.
There was some bleed-through between when they were living and the flash-sideways, including the cut on Jack's neck, and Juliet telling Sawyer they could "go Dutch."
People died at different times. Boone, Charlie, Sun, and Jin, for instance, died during island times. Kate, Sawyer, Miles, and Frank died sometime after leaving the island. Jack died on the island when his eye closed after saving the light. Because he was the character we began following first, he was the character we ended with. We saw the flash-sideways from his time perspective.
Whether they died at age 20 or age 102, they were able to find each other in the sideways. In the sideways, no matter how they looked when they died, they all remembered each other as they'd looked (age-wise) on the island.
Hurley was a great leader of the island and his decision to get them all back together in the end made everyone so happy. They were each at peace and ready to move on together to whatever was next.
Not all were, there, though. Some, like Ben, still had things to work out. Ben needed time to be with Danielle and Alex, who were not ready to move on yet. Daniel was either not dead or not ready to move on. Same with Michael and Walt. Hurley blessed them each with the choice of whether or not to move on with him and the others. Once Hurley was enlightened in the sideways, he helped Desmond to nudge people into remembering, and then let them decide what they wanted to do.
In the end, those that were ready moved on together, completely content, completely happy, and completely fulfilled. I'm sure Hurley made sure that the ones who did not come with them at that time, could join them later in blissful happiness.