what exactly is SLR, I know the net definition, but in reality what is it?
As you will know, it is meant to be a camera with a mechanism that allows the photog to see the image through the lens and thereby see exactly the photo that will be taken. Not quite so true any more as manufacturers look to find ways of getting the image to the LCD and the viewfinder.
In practice, SLRs tend to be very configurable and have a wide availability of lenses. Most keen SLR photogs will have spent far more on their lenses than their cameras and it is their lens collection that is their greatest incentive to keep them with one brand. All digital SLRs have very easy to use automatic modes and many users just use these, and many users only have one lens (unfortunately, quality-wise, this is usually the kit lens that came with the camera) so the distinction between SLRs and other cameras is not that clear.
SLRs tend to be bigger than most cameras (and can never really be described as compact) and to be at the upper end of the quality range within a brand. They are almost much more flexible in the ways they can be used with very good quality output - some non-compacts produce very good quality output, but only in good light conditions or particular circumstances.
SLRs also tend to be fast.