I'm sure you guys already know about Unallocated space, Unallocated space is free space on hard drive that can be used for store data. Unallocated space is different from Slack space. The difference, in the unallocated space the system can put files in it.
If we put some files, example 10 GB movie files, the movies data will written in Unallocated space, when we put another files example 5 GB mp3 files, then mp3 files will written in other Unllocated space. It means we have 15 GB allocated space in the hard drive.
So, what will happen when movie file is deleted..? if the movies were deleted, there will be 10 GB more Unallocated space again. However, the deleted file didn't just disappear, it just canges its status. The data of the movies is still exist in the unallocated space and will still be there until new files are written in the space. When there aren't any files overwritten the space then the movie file can be recovered with some recovery tool.
What is Slack Space..?
Slack space is the unused space between the end of the actual file and the end of the the defined data unit (cluster). Cluster is the smallest unit of storage that the operating system can deal with. When a file is written, and does not occupy the entire cluster, the remaining space is slack space.
Confusing..? ok, lets make it clear with some example. on the example Unallocated space above, we already delete 10 GB movie files, let say it that movies on sector A or cluster A, then we put 5 GB database on the sector A, so there is 5 GB unwritten space on the sector A, that's it Slack Space.
I hope this useful.. :)