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.
There are 4 or 5 "Depending on if you count Derek" types of Janet which have been established so far.
Good Janets are designed for use in The Good Place. Good Janets have a multi-stage activation process. The Good Place keeps their pre-activation Janets in the Janet Warehouse, a neutral pocket dimension beneath the Shapeless Time Void.
Two Good Janets have been seen so far. The first is Janet: the one stolen by Michael to run Neighborhood 12358W, who seems to be the most recent Good Janet put into commission. The other is the Good Janet who was next in the Janet Warehouse after the one stolen by Michael, although this one was not yet fully activated.
Bad Janets are for use in The Bad Place. We have seen several Bad Janets, though they are indistinguishable from eachother. One Bad Janet was marbleized in Chapter 24. Another's head melted and collapsed before the construction of Neighborhood 12358W, as part of a failed attempt at being good. It was implied that several Bad Janets died this way.
Neutral Janet(s) is/are for use in the Neutral Zone. This has only been seen in the Accounting Department, as Judge Gen has never displayed evidence of using any form of Janet, nor does there seem to be a need for additional Janets in the Janet Warehouse or an IHOP. It is unknown if there is more than one Neutral Janet, as she has been described by Michael's Good Janet as the "Black Sheep" of the Janet world.
In Chapter 40: A Girl From Arizona Part 1, Michael mentioned a Disco Janet, who was called "fun but a lot" and was "around for a while". It's unknown what happened to Disco Janet or if there was more than 1 of this type.
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 St. Claire.