css-floatnclear
HTML
<div id="b1"> Box1 </div>
<div id="b2"> Box2 </div>
<div id="b3"> Box3 </div>
<div id="b4"> Box4 </div>
CSS
div {
width:100px;
height:100px;
margin:10px;
line-height:100px;
text-align: center;
float:left;
}
#b1{ background-color:red;}
#b2{ background-color:green;}
#b3{ background-color:blue;clear:left}
#b4{ background-color:yellow}