Since the early days of DOS, OS/2 and Windows NT, CHKDSK has been a one of the staple utilities to keep your file systems in good, working condition. It usually springs into action automatically after a dirty shutdown to make sure that everything is okay with the file systems and makes the necessary repairs if any anomalies are detected. Plus, it can also find corrupted areas of the physical hard disk and remap affected sectors to spares in order to keep using your existing drive,
However, you may not have noticed but the old CHKDSK utility has been improved under Windows 8. The improvements are notable to the point that you may not have seen it run at all when you turn on your computer. This is because CHKDSK will execute, detect and correct file system corruption in the background. Of course, it can still be manually invoked in cases of sever data corruption if required.
This online self-healing capability allows you to keep using your computer without having to reboot or wait for the utility to finish its business with your storage media. In particular, if you use the new ReFS file system on Windows Server 2012 R2 then you can eliminate down time completely as it does not require an offline scan to repair errors.
Eventually, CHKDSK will disappear but for the time being it is still lurking in the background quietly looking after things for you.