Zipf law is frequency of words in the English language. words like "and" "but" "if "is" are right at the top of the scale while words like "onomatopoeia" are at the less used end of the scale.
PPC is a numbers game so if you bid 15c on "Sex" and get 100 clicks it'll cost ya $15. but if you use words like sexual, sexy or sexiness (less frequently clicked KW's), the $15 might last you longer and bring in a couple more joins because the KW's are more relevant to what the person is looking for (i.e. sexy chick).
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