Helper Classes are those classes , which is used to perform operations and make the code reusable.

Some of the Helper Class used in bootstrap are :-

1. Close Icon :- Bootstrap gives a general close symbol that can be utilized for releasing modals and alerts.

<div class="alert alert-warning">
    <a href="#" class="close" data-dismiss="alert">&times;</a>
    <strong>Warning!</strong> There was a problem with your network. Try Again.
</div>

https://jsfiddle.net/s0s50h7y/

2. Caret Icon :- Bootstrap provides a general caret icon to indicate the dropdown functionality and direction.

<div class="bs-example">
    <ul class="nav nav-pills">
        <li class="active"><a href="#">Home</a></li>
        <li><a href="#">About</a></li>
        <li class="dropdown">
            <a href="#" data-toggle="dropdown" class="dropdown-toggle">Services <span class="caret"></span></a>
            <ul class="dropdown-menu">
                <li><a href="#">Design</a></li>
                <li><a href="#">Development</a></li>
            </ul>
        </li>
    </ul>
</div>

https://jsfiddle.net/vq0g8gvg/

3. Quick Floats :- Used to float an element to the left or right using .pull-left and .pull-right classes. These classes included the CSS !important declaration to avoid specificity issues.

<div class="bs-example">
    <div class="pull-left">Floated to left.</div>
	<div class="pull-right">Floated to right.</div>
</div>

https://jsfiddle.net/vq0g8gvg/2/

4.  Clearfix :- This class is used to clear the float of an element. 

<div class="wrapper clearfix">
    <div class="pull-left">Float to left</div>
    <div class="pull-right">Float to right</div>
</div>

https://jsfiddle.net/vq0g8gvg/4/

5. Center Alignment of Content Blocks :- You can use the class .center-block to align the content block horizontally center

<div class="wrapper">
   <div class="center-block">Center Aligned DIV Box</div>
</div>

https://jsfiddle.net/vq0g8gvg/5/