Flare is an optical phenomenon that occurs when light from a bright source (the sun, for example) adversely affects the image by creating reflections on the glass elements of lens.
A Little More Detail:
There are two main types of flare in photography: veiling flare and ghosting flare, or ‘ghosting’. Let’s break them down.
Veiling Flare - this is where a bright source of light pulls contrast from the image and creates an unpleasant looking haze across part or all of the image. It usually occurs from a light source that is outside the lenses angle of view, shining brightly on to the front of the lens.
Ghosting Flare - known simply as ‘ghosting’, this is when the bright source of light is within the lenses angle of view. The light bounces off elements within the camera and lens, creating reflecting shapes across the image. These shapes are usually translucent, brightly coloured and mimic the shape of the light source (such as a circle) or the camera’s diaphragm. Ghosting flare is not always undesirable, and can add to the atmosphere of a shot.
To minimise flaring Canon use various lens coating substances in the construction of lens products. Additionally, the photographer should always use a Lens Hood to block unwanted spurious light from entering the lens in the first place.