Emphasis
Emphasis is a principle of art which occurs any time an element of a piece is given dominance by the artist. In other words, the artist makes part of the work stand out, in order to draw the viewer's eye there first.
In this photo of a bunch of apples, I see green apples and one red apple. The Emphasis (Contrast), can be seen in the red apple that stands out. This emphasis conveys a sense of loneliness in my opinion. This image is appealing to me because the red apple stands out.
In this photo of a house in the middle of nowhere, I see a house, grasslands and hills. The emphasis (Isolation), can be seen in the house because it stands out. This emphasis conveys a sense of mellowness in my opinion. This image is appealing to me because I don't know why anybody would live there.