Supposing that your 'trunk' is your head version and 'branches/MyBranch' is your branch folder.
In the above picture the left-hand line (9161 to 13905) is the main head line. I'd created two branches along the way from revision: 13515 and revision 13905.
One of the best things that SVN COPY does is that it DOES NOT COPY the files really - it justs links them. So that they do not take any extra space on your HD drive. If you commit to branch only the changes are stored. It is very similar to linux BTRFS file system.