<style>
.item{
width:200px;
float:left;
border: 1px solid #eee;
}

div.item_description {
max-height: 0;
overflow: hidden;
}

.item:hover div.item_description {
max-height: inherit;
}
</style>

<div class="item">
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.
<div class="item_description">
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.
</div>
</div>