Password length & complexity
A password is something that a user knows similar to a personal identification number (PIN) we use for our bank's ATM card. Coupled with user identification, it the most common form of identification and authorization mechanism implemented in web applications. Below are advantages and disadvantages on using a password as authentication mechanism.
Pros - easy to implement - inexpensive (doesn't require sophisticated hardware - easy to use (unless you forget it)
Cons - easy to guess - can be sniffed or brute forced - users tend to forget their password or list them on sticky notes posted in their monitor or under the keyboard