Hi Satou, go for DIV when you want to pack a group of element in a single box you can apply style to. For example put in a DIV all the elements you put in to create your menu. So you can apply styles for your menu by addressing all that style to the surrounding DIV tag.
Use SPAN tag when you want apply inline style. For example if you want to emphasize a phrase inside a wider text. You can count on SPAN to apply your style.
SPAN style does not create any discontinuity, for this very reason it's known as an inline element. DIV style creates a discontinuity (it starts on a new line), this is why it is called a block element.