display

значения свойства:

none
block
inline
inline-block
list-item
table
table-caption
table-cell
table-column
table-column-group
table-footer-group
table-header-group
table-row
table-row-group
inherit

css/style.css

.a {
background-color: #3BA9DE;
display: inline;
}
#div1, .b {
height: 50px;
width: 200px;
background-color: #3BA9DE;
}
.b {
display: inline-block;
}

.a {
 background-color: #3BA9DE;
 display: inline;
}

#div1, .b {
 height: 50px;
 width: 200px;
 background-color: #3BA9DE;
}

.b {
 display: inline-block;
}

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>
<div class="a">Hello World!</div>
<div class="a">Hello World!</div>
<div class="a">Hello World!</div>
</div>
<br />
<div>
<div class="b"></div>
<div id="div1"></div>
<div class="b"></div>
</div>
<br />
<div>
<div class="c"></div>
<br />
<div class="c"></div>
<br />
<div class="c"></div>
</div>
<script>
/*JavaScript*/
document.getElementById("div1").style.display = "inline-block";
/*jQuery*/
$(document).ready(function () {
$(".c").css({ "display": "block", "height": "100px", "width": "300px", "background-color": "#3BA9DE" });
});
</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>
<div class="a">Hello World!</div>
<div class="a">Hello World!</div>
<div class="a">Hello World!</div>
</div>
<br />
<div>
<div class="b"></div>
<div id="div1"></div>
<div class="b"></div>
</div>
<br />
<div>
<div class="c"></div>
<br />
<div class="c"></div>
<br />
<div class="c"></div>
</div>
<script>
/*JavaScript*/
document.getElementById("div1").style.display = "inline-block";
/*jQuery*/
$(document).ready(function () {
$(".c").css({ "display": "block", "height": "100px", "width": "300px", "background-color": "#3BA9DE" });
});
</script>
</body>
</html>