(I know this is copypasted, but that's because it explains it well. Check the sources for verification.)Earlier today, web security experts just discovered a MASSIVE security hole that they're naming "Heartbleed". Read more about it here, here or here. Heartbleed affects OpenSSL, the most popular software used to encrypt data on the Internet. It's the software that's supposed to keep your usernames, passwords, and credit card information safe from the rest of the world when you log in or buy things.
On affected servers, that means encrypted data is worse than useless: not only can attackers intercept information as if the encryption weren't there, but they can also decrypt any older data that might be sitting around. They can also use these keys to impersonate legitimate websites and fool browser security checks.
There's no way to fix this, besides waiting for servers to update their software. Security websites recommend staying away from sensitive websites for the next few days (to avoid anyone actively intercepting your information) and changing your passwords on websites that have patched the bug. This bug can affect information that's up to two years old, so it's better to be safe than sorry!
DeviantART isn't currently vulnerable to Heartbleed. Neither are Amazon, Google, Ebay, or Facebook. However, there are thousands and thousands of sites, such as Yahoo and OKCupid, that are. You should still change your passwords just in case!
You can test to see if a website is vulnerable here, and you can find a status list of popular websites here.
Excuse me while I go change my passwords. Sigh...