Ghost Whisperer is an episodic series that follows the life of Melinda Gordon, a young woman who has the ability to see and communicate with the dead. Each episode deals with one or more ghosts seeking Melinda's help in relaying a message or completing a task that will put the spirit to rest, and allow them to "cross over" into The Light, a metaphor presented by the show as its conception of Heaven.

Ghost Whisperer characters
Melinda Gordon (Jennifer Love Hewitt)
Jim Clancy/Sam (David Conrad)
Andrea Moreno (Aisha Tyler) (Season 1; Guest Season 2)
Delia Banks (Camryn Manheim) (Season 2- )
Rick Payne (Jay Mohr) (Season 2-3; Guest 4)
Ned Banks (Christoph Sanders) (Recurring Season 3; Season 4- )
Eli James (Jamie Kennedy) (Season 4- )

There are two places in afterlife, when the ghosts "cross over": The Light and the Dark Side. These two places are one of the main points of the show.

The Light
When Melinda helps ghosts cross over, they see a bright light. The Light is invisible to viewers and shown only a few times such as in episodes The Vanishing, The One, Giving up the Ghost (briefly) and The Gathering. Almost all the spirits Melinda helps in the show cross over into the light.

The Dark Side
Not all spirits cross over into the Light. Some cross over to the dark side. Spirits who are trapped in this Limbo are exposed to hate and anger.
However, some ghosts remain earthbound, whether because of an evil force, or unfinished business. Some do not even know they are dead. These spirits can reside above the surface (such as ghosts that haunt people) or in The Underneath (where evil spirits stay).