A) SLR is an acronym for single lens reflex, meaning there's one path (a single lens) for viewing and capturing the image, with a mirror (reflex) in between the lens and image sensor. When composing, the mirror is down and reflects the image coming through the lens to your eye through a pentaprism (or pentamirror), a five-sided glass prism that reflects the image in the right orientation into the viewfinder. When you release the shutter, the mirror quickly flips up and allows the image to go to the sensor. This design allows you to change lenses on the camera.
Besides the obvious benefit of accommodating various lenses, a digital SLR offers some additional advantages:
• You see exactly what's coming through the lens with an optical viewfinder (most low-priced SLRs crop the edges of the image in the viewfinder). With cameras that use one lens for image capture and another lens for the optical viewfinder, this isn't true.
• The image seen in the viewfinder is a direct view of the subject. For digital cameras with electronic viewfinders (EVF), what you see is a little monitor showing what's seen by the sensor. This view still doesn't match the optical viewfinder.
• D-SLRs are faster than most digital cameras (with minimal to no shutter lag).
• D-SLRs use larger sensors than other digital cameras, which results in images with less noise and the capability of using higher ISO settings.
On the negative side, you don't have a "live view" from the sensor on the camera's LCD (the live view lets you see exposure and color balance, for example). Since the mirror is reflecting light to the pentaprism and into the viewfinder, it's also blocking the light from hitting the image sensor when it's down. The flip side of this problem is that when you actually take the picture, the mirror flips up and you can't see the composition of the frame at exactly the moment the image is being captured.
"Body only" does mean no lens, not even a "standard" one. This is the way many SLRs have been sold for years. Why? A photographer may already have lenses from other cameras and may not want to buy another one. However, you'll often find packages that include a lens.