div, id, and class

HTML <div> Tag:

The <div> tag defines a division or a section in an HTML document.
The <div> tag is used to group block-elements to format them with CSS.
http://www.w3schools.com/tags/tag_div.asp


CSS Id and Class:
id Selector
The id selector is used to specify a style for a single, unique element.
The id selector uses the id attribute of the HTML element, and is defined with a "#".

class Selector
The class selector is used to specify a style for a group of elements. Unlike the id selector, the class selector is most often used on several elements.
This allows you to set a particular style for many HTML elements with the same class.
The class selector uses the HTML class attribute, and is defined with a "."

http://www.w3schools.com/css/css_id_class.asp