To understand the difference between a strong password and a weak one, consider this: A password of six characters that uses only numbers and letters and is not case sensitive has 10,314,424,798,490,535,546,171,949,056 possible combinations. That might seem like a lot, but to a password-cracking program, it's really not much security. A password that uses eight case-sensitive characters, with letters, numbers, and special characters has so many possible combinations that a standard calculator is not capable of displaying the actual number.
There has always been debate over how long a password should be. It should be sufficiently long that it is hard to break but sufficiently short that the user is able to easily remember it (and type it). In a normal working environment, passwords of 8 characters are sufficient. Certainly, they should be no fewer than 6 characters. In environments where security is a concern, passwords should be 10 characters or more.
Users should be encouraged to use a password that is considered strong. A strong password has at least eight characters; has a combination of letters, numbers, and special characters; uses mixed case; and does not form a proper word. Examples might include 3Ecc5T0h and e1oXPn3r. Such passwords might be secure, but users are likely to have problems remembering them. For that reason, a popular strategy is to use a combination of letters and numbers to form phrases or long words. Examples include d1eTc0La and tAb1eT0p. These passwords might not be quite as secure as the preceding examples, but they are still very strong and a whole lot better than the name of the user's household pet.