Scratches are damage to the paint caused by abrasion, when an object scores a grove.

Although swirl marks, buffer trails, micro-maring, and wash marks are all scratches, those are not typically what we mean when talking about scratches. These kinds of scratches are very light and usually only visible when they catch the light.

As long as they do not penetrate too deeply into the clear coat, they can often be polished out by cutting back the damaged paint. 

With deeper scratches, the best way to deal with them is to repaint the area, either with a smart repair or at a bodyshop, however this is expensive, and a cosmetic repair can be a good compromise, especially on areas which are low down on the car.