Because remembering many different passwords is difficult, people often reuse a small number of passwords across many different accounts, sites, and services. Today, users are constantly being asked to come up with new passwords—many people end up reusing the same password dozens or even hundreds of times.
Reusing passwords is an exceptionally bad security practice, because if an attacker gets hold of one password, she will often try using that password on various accounts belonging to the same person. If that person has reused the same password several times, the attacker will be able to access multiple accounts. That means a given password may be only as secure as the least secure service where it's been used.
Avoiding password reuse is a valuable security precaution, but you won't be able to remember all your passwords if each one is different. Fortunately, there are software tools to help with this—a password manager (also called a password safe) is a software application that helps store a large number of passwords safely. This makes it practical to avoid using the same password in multiple contexts. The password manager protects all of your passwords with a single master password (or, ideally a passphrase—see discussion below) so you only have to remember one thing. People who use a password manager no longer actually know the passwords for their different accounts; the password manager can handle the entire process of creating and remembering the passwords for them.
For example, KeePassX is an open source, free password safe that you keep on your desktop. It's important to note that if you're using KeePassX, it will not automatically save changes and additions. This means that if it crashes after you've added some passwords, you can lose them forever. You can change this in the settings.
Using a password manager also helps you choose strong passwords that are hard for an attacker to guess. This is important too; too often computer users choose short, simple passwords that an attacker can easily guess, including "password1," "12345," a birthdate, or a friend's, spouse's, or pet's name. A password manager can help you create and use a random password without pattern or structure—one that won't be guessable. For example, a password manager is able to choose passwords like "vAeJZ!Q3p$Kdkz/CRHzj0v7,” which a human being would be unlikely to remember—or guess. Don't worry; the password manager can remember these for you!