Janets are a type of afterlife beings. They exist for the purpose of serving denizens of The Afterlife and are stored in the Neutral Zone. They are neither biological beings nor robots. Janets have all knowledge of existence while in The Afterlife, and can summon objects at will.
Three "types" of Janet have been seen in the series: one designed for use in The Good Place, one for use in The Bad Place, and one for use in the Neutral Zone (specifically, the Accounting Department, as Judge Gen has never displayed evidence of using one, nor does there seem to be a need for Janets in the Janet warehouse or an IHOP), Neutral Janet. The only "good" Janet seen so far is the one stolen by Michael to run Neighborhood 12358W, it also seems to be the most recent Good Janet put into commission. It may be reasonably inferred that we have seen several "bad" Janets, though they are indistinguishable from one another. One Bad Janet was marbleized in Chapter 24.
It is unclear to what extent Derek may be considered a Janet. He was created by Michael's Janet as a rebound boyfriend and has some limited powers, including the ability to operate the Train, and the ability to summon, as he was able to summon two duffel bags of cocaine for Mindy.