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.
Janets were originally created by the Makers of Light, Darkness, and Everything, except for Derek, who was made by Janet. Presumably, Disco Janet was created by said makers, although it was never established. Since this initial creation, there have been 25 versions of Janet, the first having had a click-wheel. While they remain in The Afterlife, they are constantly being updated.
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.
Several Good Janets have been seen so far. The first is Janet, 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. Other existing Good Janets were shown in S4E8. They walked through the portals into the Judge's chamber, but she locked the portals before more could get through.
Bad Janets are for use in The Bad Place. We have seen several Bad Janets, though they are indistinguishable from each other. 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. In Chapter 47: The Funeral to End All Funerals, there are multiple Neutral Janets shown.
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 if there was more than 1 of this type. In Chapter 47: The Funeral To End All Funerals, Disco Janet briefly appears with the crowd of Janets.
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, albeit to a lesser degree. Derek has a plunger which causes him to reboot when it is pressed, similar to Janet's button. Derek also shares Janet's ability to make "Janet babies," as seen in seasons 3 and 4.