Is shutter lag the delay caused by the mirror in a DSLR camera or is it something more than this? From what I know, shutter lag is the delay caused by the mirror, whereas autofocus lag is the delay caused by the autofocus system until it autofocuses. Can you clarify this? Do I think right?
Shutter lag is a bit confusing because it doesn't have a completely fixed meaning and some changes in metering or focus can result in a difference in the amount of time it takes for a photo. Not all cameras can pre-compute focus or metering (particularly point and shoots.) Different places may use slightly different terms, though most often the shutter lag refers to the shortest period of time possible between shutter press and shutter open.
It is also a perfectly valid use to measure it as the time including other actions though since the meaning of the term is how long it takes from pressing shutter release to the shutter actually releasing. This may be particularly important when comparing a device like a P&S that doesn't allow pre-compute. It wouldn't be completely fair in that context to compare the lag of one with focusing to the other without.
It is important when comparing cameras that you take in to account how the shutter lag is being measured to make sure similar techniques were used for both.
Shutter Lag is a terrible thing! It's the length of time between when the shutter button is depressed and when the shutter actually fires. This lag time can cause many missed photos if it is long. I had borrowed someones camera to shoot kids swinging on swings and the shutter lag was so bad that when I saw what I wanted in the viewfinder and pressed the shutter, the resulting image had no actual children in it. Lower numbers are totally worth the price in my opinion, but if you shoot landscapes, and never shoot anything that moves, your priorities could be different.