Class and ids are used in CSS to give design to different types of elements of HTML if we want to give design to different elements in HTML we write the class name inside that tag element and the elements having the same class the design will apply to them.we also use ids for the same but each element has unique id so we can't give the same id to different elements.The box model is used in which the content is inside padding and padding is inside border and border is inside of margin that is the walls are the border,the space between content and walls is padding and outside the walls the margin area.