OK, let's use industrial capitalism as an example...
In industrial capitalism there is an arrangement:
Some people own the means of production but don't perform any labour themselves: the capitalists
Instead, they purchase the labour power of people who they get to work using the means of production (that are owned by the capitalists): the workers
The capitalists get to keep all the profit the workers produce. They also squeeze the workers. That's how they get rich. The workers don't get the profit they produce, that goes to the bosses. The entire relationship is essentially exploitative. And the interests of each class opposes the interests of the other class.
The entire arrangement produces two classes of people, each defined by how they relate to each other and to the means of production
Indeed, industrial capitalism itself is defined by a certain type of class relations
Every economic system is an arrangement that involves different classes of people
Karl Marx explains it better than I can. Here's a link:
Communist Manifesto (Chapter 1)
"The history of all hitherto existing society is the history of class struggles"