Suppose while viewing the output of svn diff you determine that all the changes you made to a particular file are mistakes. Maybe you shouldn't have changed the file at all, or perhaps it would be easier to make different changes starting from scratch.

This is a perfect opportunity to use svn revert:

$ svn revert README
Reverted 'README'

Subversion reverts the file to its pre-modified state by overwriting it with the cached pristine copy from the .svn area. But also note that svn revert can undo any scheduled operations -- for example, you might decide that you don't want to add a new file after all:

$ svn status foo
?      foo

$ svn add foo
A         foo

$ svn revert foo
Reverted 'foo'

$ svn status foo
?      foo

Note: svn revert ITEM has exactly the same effect as deleting ITEM from your working copy and then running svn update -r BASE ITEM. However, if you're reverting a file, svn revert has one very noticeable difference -- it doesn't have to communicate with the repository to restore your file. Which is very useful if you're working disconnected from the coffeeshop.

Or perhaps you mistakenly removed a file from version control:

$ svn status README
       README

$ svn delete README
D         README

$ svn revert README
Reverted 'README'

$ svn status README
       README