hadoop fs <args>
fs is used for generic file system and it can point to any file system such as local file system, HDFS, WebHDFS, S3 FS, etc.
hadoop dfs <args>
hdfs dfs <args>
dfs points to the Distributed File System and it is specific to HDFS. You can use it to execute operations on HDFS. Now it is deprecated, and you have to use hdfs dfs instead of hadoop dfs.