Here is an explanation from Blizzard to one person on a forum (I'll embed it below):
"A third party program is any file or program that is used in addition to the game to gain an unfair advantage. These programs may increase movement speed or teleport heroes from one place to another beyond what is allowed by game design. It also includes any programs that obtain information from the game that is not normally available to the regular player or that transmit or modify any of the game files."
Let me know of your thoughts below :)
Update: Click here to go to the forum page. It will not embed in an iframe also :(