"Cookies" are a bit of code transferred to a location in your computer that contains information about your visit. Things like pages visited, password used, preferences, etc. Mostly these have some encryption so they are not easily read.
"Cache" is where all the files (images, java scripts, etc) that get downloaded to your computer (so you can view the pages) are kept.
Delete them? What browser?
In IE 6 you select Tools - internet options, and in the window that comes up in the center section (Temporary Internet Files), click on Delete Cookies, and Delete Files.