Side channel is the security term for using side effects to glean information that someone is trying to hide.
We all use such side channels in our day to day life. We can tell someone is nervous from their body language. We might find a hiding person thanks to their protruding shadow.
Security systems generally rely on certain data, usually keys, being kept secret. Sometimes, though the key is stored securely, a side effect of the usage of the key can be used to reveal the key. A classic example of this are power analysis attacks which utilize a device's power consumption when performing cryptographic operations with a key to deduce the value of that key.
Two novel side channel attacks have been recently announced. Both of these attacks aren't too practical - but they are quite interesting.
Schneier links to a paper that shows how a mobile device's motion sensors can be used to identify a password being typed in to the device touch screen. When you press any key on the device you move the device in a particular way that is unique to that key. So if you're running my app on your device I can use the device's motion sensors to get the bank account password that you typed in another app. This attack isn't very practical yet (the information isn't accurate enough) but it's very cool.
The H sites another paper (in German) that describes how electrical power usage can be used to detect which program you're watching on TV. Previous papers showed how power usage can be used to glean information on a person's routine, but I believe this is the first time someone has used this to determine such details as which movie you've been watching. Scary.