Coupling measures how dependent one module is on another. Cohesion measures how closely related the responsibilities inside a single module are. Aim for low coupling and high cohesion.

High coupling means changes in one module ripple into many others. Low cohesion means a module handles unrelated concerns, violating SRP. A well-designed class does one thing well (high cohesion) and depends on others only through narrow interfaces (low coupling). These two properties together make systems easier to test, refactor, and extend.