What is the difference between the ‘COPY’ and ‘ADD’ commands in a Dockerfile?

You should check the ADD and COPY documentation for a more detailed description of their behaviors, but in a nutshell, the major difference is that ADD can do more than COPY:

• ADD allows <src> to be a URL
• Referring to comments below, the ADD documentation states that:

If is a local tar archive in a recognized compression format (identity, gzip, bzip2 or xz) then it is unpacked as a directory. Resources from remote URLs are not decompressed.

Note that the Best practices for writing Dockerfiles suggests using COPY where the magic of ADD is not required. Otherwise, you (since you had to look up this answer) are likely to get surprised someday when you mean to copy keep_this_archive_intact.tar.gz into your container, but instead, you spray the contents onto your filesystem.