This fires the flash upon the full opening of the first shutter curtain.
GN (Guide Number)
A measure of how powerful a flashgun is. The higher the Guide Number the more powerful the flashgun is, and measurements are usually given for ISO 100 for a 50mm lens.
Nothing to do with personal computers, PC sync sockets allow the user to connect external light sources to a camera, without needing a separate flash adaptor.
Sometimes called second-curtain sync, this fires the flash just before the second curtain closes.
A method of using a slow shutter speed with flash to obtain blur.
This lets you know the fastest shutter speed at which your camera and flashgun will sync.
TTL (Through the lens)
This is the way modern DSLRs meter a scene, by measuring the light coming through the lens off a dedicated sensor.