When an application writes a file, the data does not become permanent immediately. The write operation first moves the data into the operating system cache in RAM, where it is vulnerable to system crashes and loss of power. The second step is the transfer to the hard disk, which normally has write caching enabled. The disk acknowledges the data straight away, but keeps it in the disk write cache which is still volatile memory. The data is now safe from system crash[1. Ignoring some worst case scenarios.], but is not safe from loss of power. On a modern disk, this may be 16MB or more of data in unknown state.
As performance enhancements in ext4 have made committing data to disk a contentious issue, I’ve written a note on how different platforms handle data consistency.