Socialists see a division of society based upon the means of acquiring wealth. If you must work for a living then you are working class, if your main income is derived from the work of others then you are a capitalist. This distinction clearly exists. Even though some of us own shares, workers do not have the luxury to quit their jobs and live off investment income. When you analyze society using this class division, many problems that otherwise defy understanding have obvious solutions.
Profit is derived by owning.
Wages or salary are derived by labouring, by expending our physical or mental energy working for those who own the means of production and distribution. The owner of a particular factory may not even know that they own it. It may be just a part of an immense holding company that is administered by someone else. The workers in the factory, however, are directly connected to the production. It is vital that the capitalists pay their workers less than the value that their labour produces.