Astrology, aura reading, fortune telling, cold calling, and some personality tests all exploit the Barnum Effect: Individuals believe that personality descriptions apply specifically to them but they don't consider that the descriptions actually applies to everyone.
The term was first coined in 1956 by psychologist Paul Meehl. He compared the vague personality descriptions used in some psychological tests to P.T. Barnum, a famous showman.
The Barnum effect is also called the Forer effect. Bertram Forer used a fake psychology test on his students then gave them supposedly individualised results a week later. He asked them to rate how well it applied to them. The students rated the accuracy of the statements at an average of 4.3 out of 5.
The problem was that Forer used various bits of copy he had found in a newsstand astrology book for the fake results and that all students received the exact same list of observations.
When you know about these factors, it becomes easier to avoid falling prey to the Barnum Effect.