The Heartbleed bug is already being called one of the biggest security threats the Internet has ever seen.
It's affected many popular websites and services and could have quietly exposed your sensitive account information, such as passwords and credit card numbers, over the past two years.
Some Internet companies that were vulnerable to the bug have already updated their servers with a security patch to fix the issue. This means you'll need to go in and change your passwords immediately for these sites. Even that is no guarantee that your information wasn't already compromised, but there's also no indication that hackers knew about the exploit before this week. The companies that are advising customers to change their passwords are doing so as a precautionary measure.
Although changing your password regularly is always good practice, if a site or service hasn't yet patched the problem, your information will still be vulnerable.
Also, if you reused the same password on multiple sites, and one of those sites was vulnerable, you'll need to change the password everywhere. It's not a good idea to use the same password across multiple sites, anyway.
Click here to view a list of the most popular websites that have been affected by the Heartbleed bug.