block level formatting

I'm a little unsure of something... And sorry for the OT

#lines {
        display: block;
}

<div id="lines">
<img src="xxx" />
<img src="xxx" />
<img src="xxx" />
</div>

in this scenario is display: block saying to display div id=lines as a
block level element or to display the contents of div id=lines in this
case img as block level elements. It creates the effect I'm looking for
which is: (assuming img are lines)

_______________
_______________
_______________

as opposed to

_____________ ________________ _______________

but is this the correct usage?

Thanks!