Is there an SVN command to find the difference between two local files?

svn diff --old foo.c --new bar.c

You don't use SVN to diff files that aren't in the repository. If you are using linux then use the command:

diff file1 file2

If you are using Windows then try a tool like win merge for example

To find information about a file in the repository then use the command:

svn info file

If you want to find out more about SVN, have a look at the official tutorial.