Many websites, such as online email accounts, have a password recovery program, in case people forget their passwords. These recovery programs are usually in the form of a "secret question" -- in other words, if you forget your password you're not allowed to reset it unless you can supply the form with the correct answer to your secret question.
Because we're working with forgetful people here, the secret question is usually something easy -- such as "What city were you born in?" or "What is your mother's maiden name?"
Unfortunately, the answers to these questions are not always a secret. I first discovered this when I was about 10 years old and I had a Hotmail account. My secret question to reset my password was my dog's name -- and guess who also knew my dog's name? Well, my obnoxious older brother did, and so he quite easily "hacked" into my email account by correctly answering the secret question and changing my password.
Former VP candidate Sarah Palin also discovered this a few years ago, when her Yahoo! Mail account was hacked using information from a Wikipedia article about her. So yeah, if you're giving the correct answer to your secret question, your email password may not be very secure.
So how do you avoid that? Easy -- come up with another answer. To make it easier (for you) to remember and more difficult (for hackers) to guess, pick one answer and always use that answer, no matter what the question is.
For example, if you decide the answer to your secret question will be "pineapple," then use that answer even if the question is "What is your mother's maiden name?" Now all you have to do is remember that one virtually unguessable answer.
I like to mix it up sometimes, and pick different secret questions (if I have to choose a question from a list; if I can write my own question, I just write "the usual") every time -- but my answer is always the same.
Needless to say, this will only work if you don't forget the answer to your secret question.