undo local changes in all files
$ svn revert -R .
undo local changes in a specific file
Useful, for example, if you want a clean, atomic commit but there's a file whose differences from last revision amount to an extra blank line (or something unimportant like that which isn't a real change and might mislead people who work with you).
$ svn revert your-file-name.txt
W.I.P.: This is an unfinished work in progress.