what about multiple files and their archives in sites/default/files/[css] and /[js]? /sites/default/files/css &amp; /sites/default/files/js
my drupal sites above folders are filled with numbers of css and js files respectively and also with their archives. What are these meant to? and what they have effect on the performance of my site?

One thing i know when i explore my css in fireug, most of contents are refferd from these and even repeating and over-rided contents also appear.
How i can overcome this problem so no css over-ride? 
following is the image of css ove-riden, their actual path is sites/all/theme/themename/css but here these are from sites/default/files/css
The files are created when you enable 'Aggregate and compress CSS files.' and 'Aggregate JavaScript files.' The do have a positive effect on the performance since the browser has to do fewer http requests.

The overrides are normal as well, and they also happen when aggregation is turned off. Avoiding those means you probably have to re-write most of the css from core.