CSS

span{
    background: none repeat scroll 0 0 #F8F8F8;
    border: 5px solid #DFDFDF;
    color: #717171;
    font-size: 13px;
    height: 30px;
    letter-spacing: 1px;
    line-height: 30px;
    margin: 0 auto;
    position: relative;
    text-align: center;
    text-transform: uppercase;
    top: -80px;
    left:-30px;
    display:none;
    padding:0 20px;

}
span:after{
    content:'';
    position:absolute;
    bottom:-10px;
    width:10px;
    height:10px;
    border-bottom:5px solid #dfdfdf;
    border-right:5px solid #dfdfdf;
    background:#f8f8f8;
    left:50%;
    margin-left:-5px;
    -moz-transform:rotate(45deg);
    -webkit-transform:rotate(45deg);
    transform:rotate(45deg);
}
p{
    margin:100px;
    float:left;
    position:relative;
    cursor:pointer;
}

p:hover span{
    display:block;
}

HTML

<p>Hover here<span>some text here ?</span></p>

检查这个http://jsfiddle.net/UNs9J/1/