. is a link to "this directory"
~ is a shortcut to "my home directory"
/ means "inside this directory"
So ./a.out means the file "a.out" located inside ".", the current directory
And ~/.ddd means the file ".ddd" located inside "~", the user's home directory. You could also use $HOME/.ddd, or /home/rammoorthy/.ddd (assuming your user name is rammoorthy and your home directory is /home/raamoorthy), they're all equivalent.
The dot in front of ddd is part of the file name, it means it's a hidden file. "ls" will not show it, but "ls -a" will.
Last edited by suicidaleggroll; 10-16-2014 at 12:46 PM.