The difference between working copy and HEAD; the changes which would need to be made to what is now in the repository (HEAD), to produce your working copy:
svn diff -r HEAD --old=<file>
Of possible interest, the difference between BASE and HEAD; changes that have been checked into the repository since you last updated working copy:
svn diff -r BASE:HEAD <file>
And of course the difference between BASE and working copy; the changes you have made since you last updated working copy:
svn diff <file>
There are three versions being discussed: BASE, working copy, and HEAD.
BASE: <file> as last checked out / updated. What working copy would revert to after using svn revert
- working copy: local modifications to
<file> which has been checked out / updated as recently as BASE
HEAD: latest modifications in repository. Equivalent to BASE iff no changes have been committed since <file> was checked out / updated as working copy.