In a nutshell, if you add a certain modifier to your prompt, you can evoke a specific artist's style, an artistic genre or movement, an image-making technique, or a technical parameter. There are numerous ways to do so.

Let's take a simple prompt—archbishop --ar 2:3—as the baseline.

Now let's add a particular artist's styles. For this example, I chose a photographer, an illustrator and a fashion designer.

Note how no specifications (like age, race, or sex) change the subject depending on a particular genre or specific artist's dominant subjects.

Let's try Art Genres and Artistic Movements.

The same works with Artistic/Image-making Techniques.

What about a genre and a technique? That works too!

Finally, you can combine multiple artists' styles in one prompt.
Here are two initial modifiers:

And this is what happens when we combine them together:

Style modifiers are one of the most potent tools in the Midjourney Universe. Not only can you change the look and feel of your generation using a specific artist's style, artistic genre, or image-making technique. It is also possible to affect the contents of the scene and the main subject of your image.

This potential is one of the reasons I started the Library project. To classify the modifiers that affect Midjourney generations and provide other artists with a structured catalog of styles for them to experiment with.