In chkdsk, what is an online scan?

An offline scan is done by dismounting the volume to fix file system corruption and bring the volume back to a healthy state. Generally it is only possible with non-system volumes, and system volumes (with the dirty bit set) are scanned during boot-up before the OS is up and running completely.

Self-healing is a feature built into NTFS that fixes certain classes of corruptions encountered during normal operation, and can make these fixes while the volume is still online. If all issues that are detected are self-healed online then obviously there is no need for an offline repair. In Windows 8 the number of issues that can be handled online has been increased and hence the need for running an offline scan using chkdsk is further reduced. All this is part of the new NTFS health model:

You can also run an on-demand online scan with chkdsk /scan to identify and spot-fix file system corruption, although for major problems a full offline scan is still required.

_{Source: Redesigning chkdsk and the new NTFS health model}