Psychic Paper is paper that appears to be blank, but those with the right Gift can read the writing on it. The primary person with this gift is Lara Hodge who exhibited the gift at the age of 11. The files she keeps on the students are written on the same material and appear to contain detailed information that is novel to Hodge (i.e. she is not the only one who updates the files).
The sign up sheet for the AV club is also written on this paper and consequently all seven members are implied to have the same gift. However, the ability to read the paper does not appear to be constant (at least for Hunter) since he has been shown to be unable to read the paper in prior instances.