Run Dired on parent directory of current directory.
Find the parent directory either in this buffer or another buffer.
Creates a buffer if necessary.
Go up ARG levels in the dired tree.
The main difference between the two is one acts upon a dired-tree, the other upon a directory. A dired tree is made when you visit multiple directories which are connected to each other. The dired-tree up will work if you have previously visited that parent directory and haven't killed off that buffer. The dired-up-directory should always work since it acts upon the directory itself, think of it as doing a 'find-file on $PWD/..'.