With id and class you can specify your own selectors and then use it in your HTML elements
The selector id is used to define the style of an element.
In the definition of CSS is identified with "#"
The definition in the HMTL is identified with id
The class selector is used to define a style that will be used by a set of HTML elements, ie, to be reusable for any HTML element
In the definition of CSS is identified with "."
The definition in the HMTL identifies with class
You can also define a style for a specific item type HMTL.
For example, to define a style "red" all elements "<h1>" define this CSS:
The well will be used below: