jQuery class selector

Syntax : $('.class') 

jQuery class selectors uses JavaScript's native getElementsByClassName() function if the browser supports it. 

$('.small') // Selects all elements with class small
$('.small,.big') // Selects all elements with class small or big
$('div.small,.big') // Selects div elements with class small and any element with class big

Selects all elements with class "small" and sets 5px solid red border
<html>
<head>
    <title></title>
    <script src="Scripts/jquery-1.11.2.js"></script>
    <script type="text/javascript">
        $(document).ready(function () {
            $('.small').css('border', '5px solid red');
        });
    </script>
</head>
<body>
    <span class="small">
        Span 1
    </span>
    <br /><br />
    <div class="small">
        Div 1
    </div>
    <br />
    <span class="big">
        Span 2
    </span>
    <p class="big">This is a paragraph</p>
</body>
</html>

Selects all elements with class "small" or "big" and sets 5px solid red border
<script type="text/javascript">
    $(document).ready(function () {
        $('.small, .big').css('border', '5px solid red');
    });
</script>

Selects all elements with class "small" and all span elements with class "big" and sets 5px solid red border
<script type="text/javascript">
    $(document).ready(function () {
        $('.small, span.big').css('border', '5px solid red');
    });
</script>

Selects all elements with class small that are nested in a an element with id=div2and sets 5px solid red border
<html>
<head>
    <title></title>
    <script src="Scripts/jquery-1.11.2.js"></script>
    <script type="text/javascript">
        $(document).ready(function () {
            $('#div2 .small').css('border', '5px solid red');
        });
    </script>
</head>
<body>
    <div id="div1" class="small">
        DIV 1
    </div>
    <br />
    <div id="div2">
        Div 2
        <br />
        <div class="small">
            DIV 3
        </div>
        <br />
        <span class="small">
            SPAN
        </span>
    </div>
</body>
</html>

Selects all elements with class small and sets 5px solid red border. Notice div1 has 2 classes - small and big.
<html>
<head>
    <title></title>
    <script src="Scripts/jquery-1.11.2.js"></script>
    <script type="text/javascript">
        $(document).ready(function () {
            $('.small').css('border', '5px solid red');
        });
    </script>
</head>
<body>
    <div class="small big">
        DIV 1
    </div>
    <br />
    <div class="small">
        DIV 2
    </div>
</body>
</html>

Selects all elements that has both the classes - small and big. There should be no space between the class names. 
<script type="text/javascript">
    $(document).ready(function () {
        $('.small.big').css('border', '5px solid red');
    });
</script>

If you have a space between the two class names then we are trying to find descendants, i.e. find elements with class big that are descendants of an element with class small.
<html>
<head>
    <title></title>
    <script src="Scripts/jquery-1.11.2.js"></script>
    <script type="text/javascript">
        $(document).ready(function () {
            $('.small .big').css('border', '5px solid red');
        });
    </script>
</head>
<body>
    <div class="small big">
        DIV 1
    </div>
    <br />
    <div class="small">
        DIV 2
        <div class="big">
            DIV 3
        </div>
    </div>
</body>
</html>

Another way to selects all elements that has both the classes - small and big is by using filter method. But this approach is slower because it will first create a list of objects with class "small" and then removes elements that does not have class "big"
<script type="text/javascript">
    $(document).ready(function () {
        $('.small').filter('.big').css('border', '5px solid red');
    });
</script>

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