You are trying to install or open an app that didn't come from the Mac App Store but you got a message saying App can't be opened because it is from an unidentified developer. This is how you bypass it.
If you activated the new Inbox Tabs then after a while you find yourself not liking them that much and would like you old priority inbox back, this is what you need to do.
Pretty simple and useful tutorial, where you will open a random photograph with smoke and convert it into something really not friendly for our environment.
It's highly recommended to protect your computer against Virus, Malware, Spyware and other potential threats. Microsoft Security Essentials is one of the better, free solutions.
Exchangeable Image File Format (EXIF) data can tell you which camera was used, what focal length, ISO, exposure time was set when taking a photo. Let's see how to look for that data in Windows 7.