There IS a difference between the two, refer to the following figure from Apache's official documentation:
As we can see here, the 'hdfs dfs' command is used very specifically for hadoop filesystem (hdfs) data operations while 'hadoop fs' covers a larger variety of data present on external platforms as well. These external platforms include the local filesystem data as well.
To understand these entry-level fundamental concepts first, you should take up a hadoop tutorial. Once these doubts are clear, you can get a clearer understanding of big data analytics from a real-world scenario perspective.