The Dead on Lost sure get around much: Manifestations Guide by Ricardus Alpert

I believe that the "Ghosts" that appear on Lost belong to three categories. The first category is Manifestations of Smokey. As we've known since S3, Smokey has the ability to impersonate the dead, while possibly being able to impersonate the living as well (Walt.) We have presumably seen three (Arguably four) examples of Smokey's manifestations. Yemi, Alex and possibly Christian. The most important thing I've noticed about these manifestations is how inhuman they all seem to be. They're cold and detached, making them seem full of evil glee on occasion, such as Alex's expression when she tells Ben she "Knows it's his fault", Christian's entire demeanor during the Cabin scene, and Yemi telling Eko "You speak to me as if I were your brother."

They are not ghosts, per say, because they're not the same people they used to be once they were still alive- An entity is using their images to manipulate their loved ones, and the subtexts seem to suggest that it's a dark one.

The second category is Bodies and spirits that are controlled by MIB, and it seems to have only one example- Christian. This category is unarguably linked to the first one, but like most people, I'm still undecided on Christian's nature. I believe the most likely possibility other than him being Smokey is that he's a body or a "Spirit" that is controlled by MIB. It IS the real Christian Shepard, but he's used by MIB to do his bidding.

Now we get to the third category, which is a bit more complex. I believe that most of the "Ghosts" that have interacted with our characters on the island belong to the first two categories. The only exceptions IMO are Ben's vision of his mother who died off the island, and Hurley's visions. Why are Hurley's visions an exception? Well, mainly for two reasons. The first is that I don't think Jacob would have referred to them as a blessing if they had anything to do with MIB, and the second is that they seem to be very different from the manifestations of Smokey. They're a lot more human, and act like their past selves. Ghost Charlie knew things that Charlie did before he died, and Ana Lucia acted like you would expect her to. I don't think a Smokey manifestation would have told Hurley that "Libby says Hi."

As for Ben's mother, in Richard's first meeting with Ben, he asks him if his mother died on the island, which means that this is a phenomenon that Richard has an experience with. Only when Ben tells him that his mother died off the island, Richard starts to think that Ben is special.The Dead on Lost sure get around much: Manifestations Guide by Ricardus Alpert