.text-danger classes are used to give different colours to the text of paragraphs.
.bg-info classes are used to give different colours to the backgorunds.
.center-block classes used to give position to an element.
.clearfix class is used to clear the floats.Resize the browser to see the effect of clearfix.
.hidden classes are used to show and hide the paragraph text.
.sr-onlyclass is used in anchor tag which is only readeable by the screen reader devices.screen reader device are so usefull for blind peoples.
.close class is used to make a close button.
aria-label attribute is set to close for screen readers devices.
.text-only class replaces an element's text content with a background image and
.text-hide makes the text hidden.