HTML
<div id="bloque-a">
<div id="bloque-b"></div>
</div>
CSS
#bloque-a{
width:300px;
height:300px;
background:#ffcccc;
}
#bloque-b{
width:100px;
height:100px;
background:#ff0000;
position:absolute;
top:100px;
left:100px;
}