After partitioning, each of the partitions is further structured so that data can be saved in that partition. This structure is called a "file system." (A file system should not be confused with an operating system, which enables the starting and execution of programs.)
After a file system has been written to the hard disk it becomes difficult and even impossible in part to apply any changes (enlarge, reduce, move, or convert).
That is the reason you should decide carefully what you would like to do and whether this is realizable before partitioning a hard drive. Problems concerning this issue are discussed further down.
After the partition for a file system has been created (see further below), the file system has to be created within this partition. This is called "formatting" and is usually performed during the installation of the operating system.