I picked the right answer, but only after thinking about it for a long time - go with Books Online, or go with common sense. So I am surprised that so many people here consider this to be a simple question. However, the good news is that those people will probably be able to explain me something.
According to the Merriam-Webster English dictionary, the verb "to aggregate" is defiend as "to collect or gather into a mass or whole" (http://www.merriam-webster.com/dictionary/aggregate?show=0&t=1354796918). I checked it, just to make sure, but I am glad that the dictionary definition matches what I consider aggregation to be.
Now I have no problem with CHECKSUM_AGG and STDEVP. Both collect data from multiple rows, and generate a single value from that, be it a checksum value, or the standard deviation of the data collected. No problem. But, and here is the question I hope someone will answer me:
Exactly WHAT is aggregated by the GROUPING_ID function??