Scenario 1. You're checking your email when you notice a message that seems to be from the government, about your tax return. The news is good--you're in line for a big refund! If you just submit your banking information, they can drop the money right in there for you.
You either reply to the email, or follow a link to an official-looking site with your government tax agency logo on it (IRS, Canada Revenue Agency, HMRC, etc.). Of course, there would be nothing wrong with entering your banking information--if it was, indeed, your government. It isn't, and you could stand to lose everything on your card.
Scenario 2: You get an email about your taxes but this time, you get the email shortly after you submit. Your tax agency informs you that you can get all the payroll taxes refunded for a fee, usually one that's proportional to the amount you stand to get back (5%, 10%, etc.). They tell you the amount and ask you to wire the money. If you did so, you can kiss that money goodbye.
How to avoid: the tax refund e-mail scam is active in over 15 countries all over the world. Don’t fall for it. Take the time to read up a bit on the process of tax returns in your country, if you aren't informed already.