selector specificy

HTML

<div id="boxes" class="boxes">
    <div class="box blue"></div>
    <div class="box red"></div>
    <div class="box green"></div>
    <div class="box yellow"></div>
</div>

CSS

#boxes .box {
    width:50px;
    height:50px;
    margin:5px;
    float:left;
    background-color:black;
}

.blue {
    background-color:blue;
}
div.red {
    background-color:red;
}
#boxes .green {
    background-color:yellow;
}
.boxes .yellow {
    background-color:#fff;
}