Panel

A panel is a kind of box that is used to place your contents for better presentation. When .panel class is used, bootstrap applies some basic styles like borders and padding.

Every content should be presented inside a panel.
Example
Primary Panel

Primary title

Primary content
Success Panel

Success title

Success content
Info Panel

Info title

Info content
Warning Panel

Warning title

Warning content
Danger Panel

Danger title

Danger content 

<!--Primary-->
    <div class="panel panel-primary">
        <div class="panel-heading">
            <h3 class="panel-title">Primary title</h3>
        </div>
        <div class="panel-body">
            Primary content
        </div>
    </div>
<!--Success-->
    <div class="panel panel-success">
        <div class="panel-heading">
            <h3 class="panel-title">Success title</h3>
        </div>
        <div class="panel-body">
            Success content
        </div>
    </div>
<!--Info-->
    <div class="panel panel-info">
        <div class="panel-heading">
            <h3 class="panel-title">Info title</h3>
        </div>
        <div class="panel-body">
            Info content
        </div>
    </div>
<!--Warning-->
    <div class="panel panel-warning">
        <div class="panel-heading">
            <h3 class="panel-title">Warning title</h3>
        </div>
        <div class="panel-body">
            Warning content
        </div>
    </div>
<!--Danger-->
    <div class="panel panel-danger">
        <div class="panel-heading">
            <h3 class="panel-title">Danger title</h3>
        </div>
        <div class="panel-body">
            Danger content
        </div>
    </div>
More about panels on bootstrap.