Workers' compensation laws require employers to pay employees benefits for injuries incurred on the job, no matter who is at fault for causing the injury. In return, these laws take away the workers' "common law" right to bring lawsuits against employers for injuries caused by employer negligence.
Workers' compensation normally covers all employers, regardless of size. However, most employers carry insurance to cover workers' compensation claims, so if you file a claim, you will probably deal with an insurance company rather than your employer, because the money you receive will come from the insurance company.
Workers' compensation covers all kinds of illnesses, injuries, and disabilities that are caused by your job or made worse by your job. Workers' compensation usually pays your medical bills and reimburses you for some of your lost wages.