basic question (shared buffers vs. effective cache size)

I have a very basic question on the two parameters shared buffers and 
effective cache size. I have read articles on what each is about etc. But I 
still think I don't quite grasp what these settings mean (especially in 
relation to each other). Since these two settings seem crucial for 
performance can somebody explain to me the relationship/difference between 
these two settings and how they deal with shared memory.
Thanks much
Sally

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