Why do I "lose" space on my HD.

[Fishtalk]

Not reallly a problem but I 'd just like to know "why".
For example I clean up my HD with the disc utility then reboot.
My HD tells me I have 8.5GB of free space.... then over a period of 2 or 3 days the free space goes down to 8.2.... 8..... 7.6 ECT? This is with all the App's closed and I have'nt downloaded anything.
What's the answer please.

 

[fireshot91]

If you visit internet sites, all the history, cache, cookies, etc. will be stored.