Remaining disk space

Hi I have a Linux 120Gb aao.

Can someone explain for me remaining disk space? Thunar says I have in my whole file system 30.6Gb, yet only 9.2Gb free. In my Home properties I have 12.5Gb. When I check Filelight (graphical disk usage program) it says I have 104GB on /dev/sda1 (hard drive?) and 104Gb on /mnt/home and I'm using 96,602Mb (11% free).
Going into dev/sda1 shows it seems to count my files twice, once at mnt/ and once in home/user. Is mnt/ a partition, and is it copying my files to make a back up? Even this dosn't add up (my docs are 27.7Gb, I guess the OS and other programs are 2.9Gb), twice this would be 61.2Gb. How come I appear to have 30.6Gb on the drive, yet only 9.2Gb is free? Any help would be great.

PS I also have 246Mb which is on /dev/shm, maybe this is a partition for swap file or graphics card? It's empty at the moment.

 

I am having similar problems with the SSD drive and an SD card
8gb SSD and 16GB SDHC card.
Depending where I go, it either reports 8 GB or 21.5 gb. I don't think linpus has standardized information because its different depending on where I go to get that information.

 

That's because Linux uses different partitioning system. There is couple of partitions like:

/home - where you store your files and documents...
/user - where you installing software...
/ - where the main core of the system is sitting...
/tmp - this one is temp folder...
/opt - contains optional drivers - mostly not open source code...
/boot - this one contains Linux kernel images...

etc...

Linux is showing different "remaining" disk space depending of place where you run command... If you add-up all this numbers you going to have full HDD capacity...