Unallocated Space and Slack Space

Unallocated Space
Can be called as "free space" namely a space on hard drive that not / has not been contains data and available to be contained data

The unallocated space allow us to create partition if you need create it, also can contained data.

Slack Space
Slack space is portion of a hard drive that are not fully used by the file and maybe contained data from file that deleted before.

In picture above, in the color area is a file that in length 2700 bytes .  Because the file system cannot give the file half a cluster, it has allocated two full clusters to the file, for a total of 4096 bytes, even though the file is much smaller than that.  This means that eight sectors have been given to the file; sectors 1-5 have been used fully, sector 6 has been used partially, and sectors 7 and 8 not used by the file at all.   This means that part of sector 6 and all of sectors 7 and 8 are slack space.