Then it sounds as if you might be looking in the /tmp directory which is a top-level directory used by the operating system.
If you are using Firefox, any cached data would likely be in the hidden Firefox directory in your /home/[username] directory.
On my Debian system, that's ~/.
mozilla/firefox/ed4licj0.default$ (~ is shorthand for /home/username). Hidden files and directories have names that start with a period ( . ). This article
gives some hints on how to view them; it's not difficult once you know the tricks.
This article gives a good intro to the Linux file structure. It's really not at all mysterious, but it is very different from Windows. http://www.thegeekstuff.com/2010/09/...tem-structure/
Give it a look; it will help you understand a lot of the posts you see here.
As regards caching YouTubes, I still suggest taking a look at the thread I linked to earlier.