[R] What is the difference between prcomp and princomp?

[Peng Yu]

Some webpage has described prcomp and princomp, but I am still not
quite sure what the major difference between them is. Can they be used
interchangeably?

In help, it says

     'princomp' only handles so-called R-mode PCA, that is feature
     extraction of variables.  If a data matrix is supplied (possibly
     via a formula) it is required that there are at least as many
     units as variables.  For Q-mode PCA use 'prcomp'.


What are R-mode and Q-mode? Are they just too different numerical
methods to compute PCA?