Trivia: The film is loosely based on an ongoing series of novels by Japanese author Koji Suzuki. While the film shares the basic premise of the novels, the details are vastly different. (Notably, the books feature heavy and increasing science-fiction overtones that are absent from the films.) The American remake, "The Ring," is based more directly on this film and its sequel "Ringu 2" than the novels.
Trivia: Sadako's mother, Shizuko, was loosely based on a real person - Chizuko Mifune. Mifune was a self-proclaimed psychic. Just like Shizuko in the film, in real life Mifune took part in a press event to prove her abilities, but was labeled a fraud by reporters, and later committed suicide.
Trivia: During the climax, Sadako's unnatural movement was accomplished with a very simple in-camera effect - the actress performed the scene backwards. The footage was then reversed, which gave her movements an uncanny feeling, since they weren't quite "right." In addition, to further aid in giving her an uncanny appearance, the closeup of her eyeball in the final scenes of the film were actually shot with a male actor's eye... again to subtly make it feel not quite "right."
Answer: The other answer is not correct, although you could take it that way if you wanted. The novel upon which "Ring 0" is based was not even out at the time, nor was the prequel even planned at the time this movie came out. So that's not really the answer, although you could retroactively try to connect the two. As for the actual question: the crawling figures are typically viewed as being representations of the victims of the volcanic eruption that Shizuko (Sadako's mother) predicted. Especially as they appear right after words like "eruption" appear onscreen. Or they can be viewed somewhat more nebulously as representations of Sadako's pain, or the pain her victim's feel.
TedStixon