When we look around us, some things seem closer, some further away. Since objects in our environment look smaller when they are farther away, the easiest way to show depth is to vary the size of objects, with closer objects being larger and more distant objects being smaller. As well, we perceive objects that are higher on the page and smaller as being further away than objects which are in the forefront of a picture. When objects are partially obscured by other objects in front of them, we perceive them as further back than the covering objects. We do not see them as incomplete forms, just further back.
Atmospheric perspective uses color and value contrasts to show depth.
Objects which are further away generally have less distinct contrast - they may fade into the background or become indistinct dark areas. The foreground objects will be clear with sharper contrast. Linear perspective is based on the idea that all lines will converge on a common point on the horizon called the vanishing point. You have observed linear perspective when you notice that the lines on the highway appear to meet at a point in the distance. Artists use linear perspective to create a focal point for a picture.