Hello, and welcome to the board.
It is one of many, many Worksheet Functions, which one might think of as a formula, although, perhaps incorrectly...
Most Excel users that can demonstrate expertise with Worksheet functions will know how to use this particular Worksheet function, which I would classify as a 'general use' Worksheet function, as opposed to IRR(), which is more of a 'niche' (Finance) Worksheet function.
I would think about sitting them down in front of a PC and see if they know how to use it, and can explain it to you. If they can explain how to use it to you:
- 1) They can articulate a technical concept (good)
2) They know how to use the function
More info on VLookup() can be found here:
http://office.microsoft.com/en-gb/assistance/HP052093351033.aspx
Good luck with your search, Carolyn.