A charlatan is a quack, a person who is trying to deceive you with false claims. Beware of charlatans who try to sell you access to the fountain of youth or to a Ponzi scheme disguised as an exclusive investment fund.
A charlatan is one who pretends to possess knowledge he or she lacks. From the 16th century Italian ciarlatano "a quack," the usage of charlatan has not shifted much. Other words for charlatan are impostor, cheat, or pretender. Charlatans are marked by the elaborate schemes they cook up. Russian playwright Anton Chekhov said, "No psychologist should pretend to understand what he does not understand...Only fools and charlatans know everything and understand nothing."