\hfil \hfill \vfil \vfill
The TeX primitives \hfil, \hfill \vfil, \vfill, \hfilneg, \vfilneg are respectively equivalent in their effect to the command sequences:
\hskip 0pt plus 1fil \hskip 0pt plus 1fill \vskip 0pt plus 1fil \vskip 0pt plus 1fill \hskip 0pt plus -1fil \vskip 0pt plus -1fil
In other words, they provide infinite horizontal and vertical glue, where \hfill and \vfill are infinitely larger than \hfil and \vfil respectively.
Please note that while you can use
\hskip 0pt plus 1filll \vskip 0pt plus 1filll
there is no \hfilll or \vfilll.
The glue produced by these commands never shrinks.