There have been various changes made to the chkdsk and NTFS health model. Here's an overview:
Online self-healing: The NTFS self-healing feature was introduced in Windows Vista (and in Windows Server 2008) to reduce the need to run chkdsk. Self-healing is a feature built into NTFS that fixes certain classes of corruptions encountered during normal operation, and can make these fixes while still online. In Windows 8 the number of issues that can be handled online has been increased and this has reduced any further need for chkdsk.
Online verification: Some corruptions are intermittent due to memory issues and may not be a result of an actual corruption on the disk; so a new service to Windows 8 has been added, called the spot verification service. It is triggered by the file system driver and it verifies that there is actual corruption on the disk before moving the file system along in the health model. This new service runs in the background and does not affect the normal functioning of the system; it does nothing unless the file system driver triggers it to verify a corruption.
Online identification and logging: When an issue is verified, this triggers an online scan of the file system, which runs as a maintenance task in the file system. In Windows 8, scheduled tasks that are for the maintenance of the computer run only when appropriate (during idle time, etc.). This scan can run as a background task while other programs continue to run in the foreground. As the file system is scanned, all issues that are found are logged for later correction.
Precise and rapid correction: At the user or administrator’s convenience, the volume can be taken offline, and the corruptions logged in the previous step can be fixed. The downtime from this operation, called “Spotfix,” takes only seconds, and on Windows Server 8 systems with cluster shared volumes, this downtime has been eliminated completely. With this new model, chkdsk offline run time is now directly proportional to the number of corruptions, rather than being proportional to the number of files as in the old model.
There have been various other changes made to implement better manageability as well, and the new file system health model basically works like this:
Check the Source for more details, 'cos otherwise it's just gonna end up as a complete copy-paste job, and with articles like these it's really difficult to reword everything.