How Does third Party Analyse the Web Traffic ??

Recently I have seen a Site, which analyzes the site Traffic! Can any
one explain what is the technalities behind it ?

Actually, they say that they will place a code on our web page. That is
ok that that will place javascript (src) code and place the file on
their server and can can check their web logs when that particular
files is being called.. this is ok

But what they say that they can know which link has been clicked among
the dozen of links present on the site, I am not getting any logical
connection between this.. Please help in understanding the funda behind
this.. and these site are saying that they will not going to edit the
url's .. right.. then hOw will they come to know that which link has
been clicked ??

Please help me uncovering the logic behind it ..

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Thanks & Regards

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