Usually in reference to telephone systems, 411 connects the caller to the Information operator. Dial 411 if you need to know an Area Code for a certain City, or someone's telephone number.

Also used as a slang replacement for the word "information", or any request thereof, such as in the examples, "What's the 411?" or "Give me the 411."

A generic, country-wide replacement for the old Information phone number <areacode>-555-1212. 411 has the annoying feature that it cannot be dialed if you do not have long distance service¹.

See Also: 412, 410, 419

¹ Yes, some people are unwilling to pay $100 deposits so they can have long distance service. Besides, how often do you call long distance? I rarely need to.