Something I see ppl define the variable like this:

b2World *_world;
b2Body *_body;
CCSprite *_ball;

instead of

b2World *world;
b2Body *body;
CCSprite *ball;

I familiar with the second one, but not the first one. So, I checked the Wikipedia about naming convention:

Names beginning with double underscore or an underscore and a capital letter are reserved for implementation (compiler, standard library) and should not be used (e.g. __reserved or _Reserved).

So, is that any special meaning which is start with "_"?

The wiki page.