What is the difference between the link and ln commands?

link used solely for hard links, calls the link() system function and doesn't perform error checking when attempting to create the link

ln has error checking and can create hard and soft links

The link command creates a hard link to a file.

According to the manual page, it is the same as:

$ ln --directory --no-target-directory FILENAME LINKNAME

See man link or info coreutils 'link invocation' for more details.