margin-bottom

значения свойства:

0
auto
size
%
inherit

css/style.css

* {
margin: 0;
padding: 0;
}
.wrapper {
width: 900px;
height: 150px;
margin: 10px auto;
border: 1px solid #808080;
}
.a {
margin-bottom: 10px;
}
#div1, .a, .b {
width: 300px;
height: 50px;
background-color: #7092BE;
}
.c {
width: 300px;
height: 50px;
background-color: #852E76;
}

* {
 margin: 0;
 padding: 0;
}

.wrapper {
 width: 900px;
 height: 150px;
 margin: 10px auto;
 border: 1px solid #808080;
}

.a {
 margin-bottom: 10px;
}

#div1, .a, .b {
 width: 300px;
 height: 50px;
 background-color: #7092BE;
}

.c {
 width: 300px;
 height: 50px;
 background-color: #852E76;
}

index.html

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<link href="css/style.css" rel="stylesheet" />
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
</head>
<body>
<div class="wrapper">
<div class="a"></div>
<div class="c"></div>
</div>
<div class="wrapper">
<div id="div1"></div>
<div class="c"></div>
</div>
<div class="wrapper">
<div class="b"></div>
<div class="c"></div>
</div>
<script>
/*JavaScript*/
document.getElementById("div1").style.marginBottom = "15px";
/*jQuery*/
$(document).ready(function () {
$(".b").css({ "margin-bottom": "20px" });
});
</script>
</body>
</html>

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<link href="css/style.css" rel="stylesheet" />
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
</head>
<body>
<div class="wrapper">
<div class="a"></div>
<div class="c"></div>
</div>
<div class="wrapper">
<div id="div1"></div>
<div class="c"></div>
</div>
<div class="wrapper">
<div class="b"></div>
<div class="c"></div>
</div>
<script>
/*JavaScript*/
document.getElementById("div1").style.marginBottom = "15px";
/*jQuery*/
$(document).ready(function () {
$(".b").css({ "margin-bottom": "20px" });
});
</script>
</body>
</html>