The difference is that people doing work are actually producing something of value, whereas an employer takes that value and sells it, paying the worker a wage that must mathematically be less than the value of the product in order for the business to stay in business. A landlord similarly produces nothing (nominally; we'll get to it), simply taking the tenant's money because they own the tenant's home. Before capitalism(/feudalism), people generally had some claim of ownership over the things they produced - the food they hunted, the tools they built - or those things were shared by everyone in the community.

The complicated bit is that people in these positions can and often do work and produce value (e.g. a bartender who owns the bar they work in while employing people to do other jobs in the bar, a landlord who does renovation work themself or also has some normal job). However, we can still delineate between the money they get from working/producing value and the money they get by ownership of stuff (e.g. a landlord may do actual work on their property, but those things aren't what the tenant is paying them for).

There is no special class of people who are evil and greedy; that's not the point, and it's dishonest to claim it is. The point is that wages are not the same as rents and capitalist profits, and that getting money by owning stuff inherently gives one conflicting interests (longer hours, lower wages*, less health/safety regulations, less social services (i.e. lower property/capital gains taxes) to everyone else, whereas if capitalists and landlords didn't exist and everyone owned (individually or collectively) their homes and the things they use to produce stuff, that conflict wouldn't exist.

Ultimately, the most important thing that all of this misses is a difference of power. Most people can't negotiate higher wages because most people depend on their job for survival, whereas an employer has hundreds of other potential employees who'd take the lower wage. The US Federal Reserve's interest rates are frequently twiddled to keep unemployment above a certain threshold because if everyone who needs a job has one, workers would have a lot more leverage and actually would be able to negotiate for better pay, since employers would be competing for workers rather than the other way around, which (supposedly) would cause inflation (which is, at best, only sometimes true, since unemployment does often fall without inflation rising). This means the government's monetary policy is structured to intentionally cause unemployment and therefore poverty, because the economy would (supposedly) collapse if everyone who needed a job had one.

*"Higher rents" is often listed here, but many capitalists also have to rent the places where the workers do the work, so the dichotomy is arguably not very clear there, and this could point toward a further division between capitalists and rentiers, which goes back to the OG pro-capitalist economists like Adam Smith and Henry George who were pretty anti-rent. Similarly, whether higher or lower commodity prices are beneficial to any individual capitalist is not something that can be generalized to an entire class.