Edit in JSFiddle

.moncube
{
width:100px;
height:100px;
background:red;
position:relative;
animation:mymove 5s infinite;
-webkit-animation:mymove 5s infinite; /* Safari and Chrome */
}

@keyframes mymove
{
from {top:0px;}
to {top:200px;}
}

@-webkit-keyframes mymove /* Safari and Chrome */
{
from {top:0px;}
to {top:200px;}
}

<div class="moncube"></div>