A password is a form of secret authentication data that is used
to control access to a resource. The password is kept secret from
those not allowed access, and those wishing to gain access are
tested on whether or not they know the password and are granted or
denied access accordingly.
The use of passwords goes back to ancient times. Sentries guarding
a location would challenge for a password. They would only allow a
person in if they knew the password. In modern times, passwords are
used to control access to protected computer operating systems,
mobile phones, cable TV decoders, automated teller machines (ATMs),
etc. A typical computer user may require passwords for many
purposes: logging in to computer accounts, retrieving email from
servers, accessing files, databases, networks, web sites, and even
reading the morning newspaper online.
Despite the name, there is no need for passwords to be actual
words; indeed passwords which are not actual words are harder to
guess (a desirable property), but are generally harder for users to
remember (an undesirable property). Note that password is often
used to describe what would be more accurately called a pass
phrase. Passcode is sometimes taken to imply that the information
used is purely numeric, such as the personal identification number
(PIN) commonly used for ATM access. Passwords are generally short
enough to be memorized.
Source @ http://en.wikipedia.org/wiki/Password
