On a single System
CSS and JS files cache are saved in browser cache. You can force browser to reload all CSS and JS cache by pressing "CTRL + F5". By pressing this, all CSS and JS files reloaded again on browser and it takes more time to load.
On all customer's System
If you have a website that customers are using and you want to change the CSS in all customer's end. You can pass parameter in your CSS file.
Ex. Suppose you are using stylesheet.css in your header file, it should be included in header.tpl in common folder in your theme template folder as
<link href="catalog/view/theme/default/stylesheet/stylesheet.css" rel="stylesheet">
Whenever you make some changes in your CSS files, just change the parameter in header.tpl as
<link href="catalog/view/theme/default/stylesheet/stylesheet.css?version=1" rel="stylesheet">
since the file URL is changed, browser will have to reload this file on all customer's system. So your changes will appear on simple reload of web page.
Next time you change the stylesheet.css file, just modify header.tpl to
and so on.
Hope, it helps.