At the beginning of Ways of Seeing, John Berger writes that

how we see things is affected by what we know or believe.

He uses as an example, the idea that a person from the Middle Ages with a strong conception of heaven and hell would look at fire differently than we do today. He might see hell, while we see warmth or the chance to roast marshmallows.

What Berger means by ways of seeing is that our way of seeing objects is culturally determined. He uses as an example masterworks of art. At one time, they were meant to display the wealth and power of the owners. They were not meant to evoke envy, because in a static, highly stratified society based on hierarchy, such as eighteenth century England, ordinary people had no expectation that they could have or be what the aristocracy did.

This changed with the advent of mass consumer culture in the twentieth century. At that point, masterworks of art were used to sell products anyone could aspire to own, such as perfume or hairspray. The same artworks, now used to advertise products, were seen differently because the culture had changed. Now motifs from great art sell jewelry or deodorant.

Also, with the advent of cheap reproduction, we stopped seeing great art as something impossible to possess. Actually, we can possess it: we can buy endless reproductions, for example, of the Mona Lisa, in prints, post cards, on tee shirts, and on coffee mugs.

Art is now deployed, Berger argues, to sell all of us the illusion we can all have the good life. This because our culture has trained us to see it differently than in the past.

The underlying premise of Berger's concept of a "way of seeing" is the dynamic interrelationship between seeing the world--which comes before language about the world--describing the world, an action that creates an ideology that reverses on itself and effects how we see the world. In other words, we, as thinking humans, do not remain neutral about what we see and the way we describe it. Instead, we mold our thoughts by what we see, mold our thoughts even more by the words and conceptualizations we develop to describe what we see, then are paradoxically molded by what we say about what we see.

When this paradoxical dynamic interrelationship between seeing, saying, and conceiving of a concept is applied to looking at art, we bring tour paradoxical dynamic ideology with us to that which we look at. Thus art (or anything) that we see is seen in a way particular to our way of thinking that has been molded by what we originally see (and continue seeing) and describe and the words we originally use to describe (and continue to use) what we see. Nothing we see can be "neutral" after we create our inner ideologies around language and words. Everything moves dynamically as part of the interrelationship that is built into an ideology of seeing, or, what Berger calls a way of seeing.

There are other elements to his theory, such as cultural mystification, which you can explore in a critical essay and a summary.

First, some background information.The BBC produced Ways of Seeing as a four-part series in 1972. John Berger and Mike Dibb produced it. Later this work became a book - seven chapters. Three of them are visual and the other four are essays.  

As you can imagine, this work concerns the topic of ways of seeing. Art, sociology, cultural studies, and history are commingled. Now to your question.

Berger argues that Western art embodies hidden ideologies in its visual images. In other words, art, even though it does not have words contains a cultural message. Berger, therefore, removes the veil from our eyes to show that art is a product of an elite way of seeing. Art embodies the power structures of those who have power and seeks to perpetuate them. From this perspective, we need to ask who commissioned the work of art, who is it meant to see it, and what are the hidden messages. 

As an example, the second essay deals with the art of female nudity. Berger argues that these nudes are not realistic. Rather they are the idealizations of men. In this sense, these works of art objectify women. The subject is obviously men, that is, the ones in power. So, we can say that in females nudes, we embody a distinct male way of seeing. 

Finally, I will attach an article that is an up-to-date usage of Berger's insights.