By and To, what is the difference?

You will notice that each Action has a By and To version. Why? Because they are different in what they accomplish. A By is relative to the current state of the Node. A To action is absolute, meaning it doesn't take into account the current state of the Node. Let's take a look at a specific example:

auto mySprite = Sprite::create("mysprite.png");
mySprite->setPosition(Vec2(200, 256));

// MoveBy - lets move the sprite by 500 on the x axis over 2 seconds
// MoveBy is relative - since x = 200 + 500 move = x is now 700 after the move
auto moveBy = MoveBy::create(2, Vec2(500, mySprite->getPositionY()));

// MoveTo - lets move the new sprite to 300 x 256 over 2 seconds
// MoveTo is absolute - The sprite gets moved to 300 x 256 regardless of
// where it is located now.
auto moveTo = MoveTo::create(2, Vec2(300, mySprite->getPositionY()));

// Delay - create a small delay
auto delay = DelayTime::create(1);

auto seq = Sequence::create(moveBy, delay, moveTo, nullptr);

mySprite->runAction(seq);