In HARD mount :
If the NFS file system is hard mounted, the NFS daemons will try repeatedly to contact the server. The NFS daemon retries will not time out, will affect system performance, and you cannot interrupt them
If you just mount a file system without specifying hard or soft, the default is a hard mount. Hard mounts are preferable because of the stateless nature of NFS.
If a client sends an I/O request to the server (such as an ls -la), and the server gets rebooted, in client machine the process will keep on running.
This preserves data transfers in the event of a server failure
In SOFT Mount :
A soft mount allows the client to stop trying an operation after a period of time. If the NFS server goes down at the time of data transfer , it will alert and the process will do down.Thsi may cause the data corruption.
A soft link will return with an error and fail.
you should only use soft mounts in the cases where client responsiveness is more important than data integrity.
In another word ..soft mount will allow automatic unmount if the filesystem is idle for a specified time period