…and do not conjure. (Leviticus 19:26)
This prohibition includes at least two aspects. First, it prohibits ascribing auspicious times to activities such as, “If you take the 7:00 train you will make money today but if you take the 8:00 train, you will lose money.” (Again, the relevance of the timing makes a difference. “Buy today, before the stock splits” is relevant time and not prohibited under this mitzvah.) The second aspect of this mitzvah is a prohibition against making illusions in an attempt to trick people into thinking you have supernatural abilities.
The obvious question is magicians who perform acts of prestidigitation. Such sleight of hand is not prohibited as they are only performing for the entertainment of the audience. If the intention is solely to impress the audience with his skills, the magician breaks no prohibition. If, however, he lies and claims to possess supernatural powers, he would then be in violation of this mitzvah. (See Igros Moshe Yoreh Deah 4:13.)
The reason for this mitzvah is that tricksters can (and historically do) lead people astray. Claiming to have special abilities and, presumably, a direct line to God only causes to lead people away from the real deal.
This mitzvah applies to both men and women at all times and in all places. It is discussed in the Talmud in tractate Sanhedrin on pages 65a-b. This mitzvah is codified in the Shulchan Aruch in Yoreh Deah 179. It is #32 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #166 of the 194 negative mitzvos that can be fulfilled today as listed in the Sefer HaMitzvos HaKatzar of the Chofetz Chaim.