There is no difference in meaning. Some might say "yes" is more formal, but I would personally use "yeah" even in formal company, or speaking on TV. Listen lots to the way people around you speak to decide which you would rather use, but both are the same really.

You may check them out through the following references: http://www.wordreference.com/definition/yes http://www.wordreference.com/definition/yeah === Good weekend. / 祝 週末愉快。