svn diff -r 2:6 --summarize - shows summary of changed between revisions 2 and 6 in the current directory
svn diff --old=https://win2008r2-vm/svn/ReleaseTest/trunk --new=https://win2008r2-vm/svn/ReleaseTest/branches/R01 - this compares the R01 branch with trunk, but trunk in its current state, however add "@2" to the end of the --old would compare to an old revision, 2 in this case, which is back where the branch was made
You can do the same as
svn diff --old=https://win2008r2-vm/svn/ReleaseTest/trunk@2 --new=https://win2008r2-vm/svn/ReleaseTest/branches/R01 --summarize from the Repository Browser. Select the branch, right click it and choose "Mark for comparison", then find "trunk", right click and select "Compare URLs", this will work with HEAD in both cases but you can change this in the diff dialog.