About cache, cookies, and history
Each time you access a file through your web browser, the browser caches (i.e., stores) it. Thus, the browser doesn't have to newly retrieve files (including any images on the page) from the remote web site each time you click Back or Forward. You should periodically clear the cache to allow your browser to function more efficiently.
A cookie is a file created by a web browser, at the request of a web site, that is stored on a computer. These files typically store user-specific information such as selections in a form, shopping cart contents, or authentication data. Browsers will normally clear cookies that reach a certain age, but clearing them manually may solve problems with web sites or your browser.
A browser's history is a log of sites that you visit. When you press a browser's Back button, you are moving back one entry the history log. Browsers will normally clear history at regular intervals, but you may want to clear it manually for privacy reasons.
Internet Explorer 8 and above
From the Safety menu in the upper right, click Delete Browsing History... .
Deselect Preserve Favorites website data, and select Temporary Internet files, Cookies, and History.
Click Delete.
Internet Explorer 7
From the Tools menu in the upper right, select Internet Options.
Under "Browsing history", click Delete... .
To delete your cache, click Delete files... . To delete your cookies, click Delete cookies... . To delete your history, click Delete history... .
Click Close, and then click OK to exit.
Firefox 3.5 and above for Windows
From the Tools menu, select Clear Recent History... .
From the Time range to clear: drop-down menu, select the desired range; to clear your entire cache, select Everything.
Click the down arrow next to "Details" to choose what history elements to clear (e.g., check Cookies to clear cookies). Click Clear Now.
More here: http://kb.iu.edu/data/ahic.html