What does the woman behind the wallpaper represent, and why does the narrator come to identify with her?

The narrator sees the woman in the wallpaper as a prisoner, just like she is a prisoner in her room.  She can't do anything that her husband and sister-in-law don't approve of, nor can the woman in the wallpaper escape.

While in the room, the narrator's depression becomes deeper and more advanced toward mental illness. Her desire to free the woman in the wallpaper is a cry from within trying to emerge from the psychosis that envelopes her.  She can't seem to shake free of the melancholy that dominates her life.

"Despite her efforts, however, she cannot remove it all. In her desperation, she considers committing suicide but decides that this would be "improper and might be misconstrued." She begins circling the room, following the pattern of the wallpaper, in essence becoming the woman inside, trapped in an endless maze. John breaks open the door to see his wife creeping along the wall and faints. The narrator only laughs. His slumped body is blocking her path, and she is forced to creep over him each time she circles the room."