NFS refer to Network File System. It provides a shared directory to a number of machines that can access the directory as good as a directory on local file system. On the other end, HDFS is a distributed file system, in which different machines grouped for storing the data in a distributed way.

Following are some of the key differences:

1. In NFS, All data belonging to an entity(file or set of files) stored on a single machine and thus require a dedicated hardware. In HDFS, the same data is stored in a distributed way on set of commodity hardwares.
2. HDFS allows parallel processing of data through MapReduce. Since data on NFS is stored on a single location, it has beed read sequentially.
3. HDFS replicates the data and thus provide fault tolerance. Since data in NFS is stored on a single machine, it is difficult to access/restore if the machine goes out of network.

Hope this would help.