Wikipedia is your friend (http://en.wikipedia.org/wiki/Datum). Basically it's a way of getting a nominal 'centre' of the earth for the purpose of marking lines of latitude and longitude. Because the earth isn't a perfect sphere, this is harder than it sounds...
That is, I did go and check on wikipedia when I read your first post, I'm sorry, I forgot to say so.
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That is, I did go and check on wikipedia when I read your first post, I'm sorry, I forgot to say so.