A work ethic is a set of values people have about the benefits and importance of working hard and being productive. Values are subjective, so a coworker doesn't necessarily have a bad work ethic if his opinions about working aren't in line with yours. However, you should address a fellow employee's work ethic if he's making it difficult for you to complete your job duties. Address the problem with the employee directly first, but sometimes you have to involve a manager.
Avoid the temptation to wait for a coworker to figure out that his bad work ethic is affecting you. Speak to the coworker in private, and explain the problem by giving specific examples of how his failure to complete work hampered your ability to get your job done. Explain the problem with a teamwork perspective, pointing out how he and others have an important role to fulfill in the workplace.
Find out if your coworker understands how to complete his assigned tasks when you discuss work-ethic problems with him. Employees sometimes get duties from managers that they don't have the skills to fulfill, so they avoid those duties. Recommend that a coworker ask a manager for guidance or training on how to complete tasks he doesn't understand. Help him yourself if you can, but don't do his job for him.
Bear in mind that a coworker might not be getting his job done because he has personal problems that are distracting him. Don't feel compelled to take on a coworker's problems, but you can show understanding by giving him some slack on the job while he sorts out his troubles. In such cases, ask him to consider whether taking time off from work would be beneficial in tackling his problems.
Tell your manager about the problems a coworker's bad work ethic is causing if your other efforts to help him fail. Don't make the issue personal when you tell your manager about the matter. Present your manager with business-related reasons the coworker's poor work habits are affecting the workplace. Consider things such as whether the coworker's behavior is creating a backlog of work for you and others.