Outline is used to make an Element visually more attractive and to look different than other adjacent Defined Elements.
CSS provides an outline property to design an outline design of elements. By this property you can define the outline of the element’s color, style and size.
Outline is a line outside the outside of an element. This line is outside the border. The boundary of any element is part of the same element. But the outline is not part of the element.
If you set the background of an element, then you will have a background color show within its border. This part is not filled in the part of the border outside the border. Because the outline is not part of an element.
The main difference between the border and the outline is that if you want to define the border of one of the sides. But the outline is always define for all sides only.