The article examines different types of coupling and what they look like in Rails:

Coupling refers to the degree to which components in your program rely on each other. You should generally seek to minimize this property, though you’ll see it’s impossible to eliminate entirely.

• Pathological coupling: one class reaches into another and changes instance variables.
• Global coupling: Two classes that rely on some global state.
• Control coupling: a parameter is passed to a method that determines the control flows in the method.
• Data coupling: a parameter is passed to a method that does not impact the control flow in the method.
• Message coupling: Jim Weirich calls this “Connascence by Name”. The least costly type of all couplings. This is the one we should strive for.

Link: Types of Coupling via robots.thoughtbot.com