You see the terms “flare” and “ghosting” bantered around a lot, particularly when reading the photography web about lenses. Put simply, these terms describe reflections that occur inside lens elements within lenses and filters, and reflections between the lens and the imaging sensor.
Flare, sometimes called “veiling glare,” is a tendency for light to fill the frame and obscure the subject, and ghosting is the appearance of objects in the frame often shaped like blobs or like the shape of the lens’ aperture.
Some lenses are resistant to flare, while others will flare with little provocation. Generally, but not always, single focal length (“prime”) lenses flare less than zoom lenses (which have more, often many more, lens elements inside), and generally, top quality lenses flare less than cheap lenses, and “fast” (large aperture) lenses flare more than lenses of more modest maximum apertures.
Flare and ghosting are almost always a consequence of having bright light source in the frame; a window, a street light, the sun, or headlights, for example. Lens hoods, which I always use to protect my lenses, are not very effective at controlling flare because they are often not quite large enough, and you will often see me shading my lens with my hand when shooting into a bright light source.
Flare and ghosting are reduced by using smaller apertures.
Finally, flare and ghosting, while referred to as an aberration in technical talk, can often contribute to the success of an image. Some fine art photographers use older lenses, for example, to convey a sense of “vintage” in their images. I know a wedding photographer whose entire look is based on flare at sunset. In my own work, I often use flare and ghosting to convey a sense of brightness that might not otherwise be expressable.