MKDIR [drive:]path
MD [drive:]path

If Command Extensions are enabled MKDIR changes as follows:

MKDIR creates any intermediate directories in the path, if needed.
For example, assume \a does not exist then:

mkdir \a\b\c\d

is the same as:

mkdir \a
chdir \a
mkdir b
chdir b
mkdir c
chdir c
mkdir d

which is what you would have to type if extensions were disabled.