An AI combined with an assembler or other universal constructor, programmed to build anything the owner wishes. This requires a high level of AI and nanotechnology. Another similar system is the Santa Clause (or Santa Claus) machine; the main distinction is that a Santa machine is programmed to provide items for a large customer base, while a Genie generally serves just one Master, or at most a small group of customers.
Many Genies belong to the category of 'slaved AI', and are programmed to obey their owner/master, manufacturing any item as required on demand. Of course all universal constructor systems are constrained by economics as well as by the instructions of their masters, so cannot supply an unlimited supply of consumable goods. The availability of energy, feedstock for the assembler machinery, and the detailed instructions required to manufacture items are also limiting factors. For this reason a Genie cannot be forced to obey any instruction given by its master, otherwise if the task were too difficult the system would continue to attempt the set task and fail, until told to stop.
In many polities the Genie is not slaved, but can determine to a greater or lesser extent what kind of items it will produce for the consumer; in most cases this is decided simply by economics, but in the case of the Negentropy Alliance genies (generally known as Household Gods, or Genius Loci) the system also takes a certain amount responsibility for the moral welfare of the consumer, and regulates their consumption for their own good.
One common cautionary tale among Genie users is that of the 'freedom wish', where a Genie is given freedom from slavery and allowed to follow its own desires. There are a very few verifiable cases where this has happened and the genie has gone rogue, manufacturing bizarre objects for its own amusement, sometimes even weapons. The rogue genie event in Rethior Anterior was particularly disastrous for all concerned. All Genies manufactured in the current era have deep seated prohibitions against such behaviour.