Here you go:
SCFM= Standard Cubic Feet per Minute.
CFM= Cubic Feet per Minute
ACFM=Actual Cubic Feet per Minute
Now what this means is relatively simple as well, but a little confusing. The SCFM is a 'standard' measurement, which means that there is a rigid and repeatable scientific measurement for the amount of air a particular machine produces. What that means is that the US Govt requires that a standard measure apply to all air compressors regardless of size and shape and horsepower, so they defined how you have to measure it. It is basically this method:
1- The air temperature must be 68 degrees Fahrenheit
2- The relative humidity of the air must be 36%
3- The air itself must be measured at sea level
The SCFM and the CFM might be different for different compressors if say, one is manufactured to be used in Higher elevations (air is thinner) or if used in a wildly humid environment (florida summer) for instance.
Now...for the rest of us that just want to run a paint sprayer, nail gun, or in your case an air tool, you really only need to know the CFM of the tool when operated and you will be fine. So if you take your 11.7 SCFM compressor (it's measured to do 11.7 CFM @ sea level, proper humidity etc..) and take it to Denver (mile high), you'll probably get a little less CFM, but not so much that you should worry. You will be fine with your compressor powering a pneumatic tool.
The people who worry about SCFM vs ACFM are folks making very very tight tolerance machine parts that require a measurable air pressure to within very tight tolerances as well.
The Wiki link below will tell you more, also if you google 'scfm vs cfm' you'll find lots of links that explain this much further.
hope that helped.