Cache memory ,on the client side is where cookies r stored.
Some web applications make use of such cookies to identify
the  users,hence forms an important part of web testing.

Cache is in Webapplications like Jobportals.Mainly What is 
thePurpose to use this is for the security part and if all 
the users are Submiting at a time in teh Applications then 
the hits of the Server willbe high.to protect this Capche 
is Used.

As i am from electronics baground i will explain it in that 
language. 

you know hard disk is slowre memory than processor due to 
that with ref. to paging concept we use RAM to increase 
speed of our computer but this RAM is also slowre memory 
than processor due to that for syncronization purposr the 
use one fastes memory called Chache memory. chache is 
nothig but a chip (IC).

suppose you want to access the 3 page and 4th line of that 
page and on next event you want 25 line in that case this 
chache memory make buffering of that total page and provide 
it to processor for fast access. 

as you test cookies at the time of testing web application 
is nothing but address of that page which is buffered in 
the chache memory at the time page opening and testing of 
that page.