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SOLID Principles

The SOLID principles are a set of basic principles for designing OO programs. The name itself is a acronym, with each of the five principles named after one of the letters: Single responsibility, Open/closed, Liskov substitution, Interface segregation, and Dependency inversion. The principles act as a set of guidelines to help you implement code that is easy to maintain and extend over time. The SOLID acronyms was coined by Robert C. Martin (a.k.a. Uncle Bob).

Each of the principles corresponds to a set of potential code smells that can exist in your code, and they offer a route out of the problems that they cause.

Initial	Acronym	Concept	Author
S	SRP	The Single Responsibility Principle A class should have only one reason to change.	Robert C. Martin
O	OCP	The Open Closed Principle Software entities should be open for extension, but closed for modification.	Bertrand Meyer
L	LSP	The Liskov Substitution Principle Methods that use references to base classes must be able to use objects of derived classes without knowing it.	Barbara Liskov
I	ISP	The Interface Segregation Principle No client should be forced to depend on methods it does not use.	Robert C. Martin
D	DIP	The Dependency Inversion Principle Depend upon abstractions. Do not depend upon concrete classes.	Robert C. Martin

Other important principles:

• Cohesion: It refers to how closely all the routines in a class or all the code in a routine support a central purpose –how focused the class is. Classes that contain strongly related functionality are described as having “strong cohesion”, and the heuristic goal is to make cohesion as strong as possible. Cohesion is a useful tool for managing complexity because the more that code in a class supports a central purpose, the more easily your brain can remember everything the code does.
• Coupling: It describes how tightly a class or routine is related to other classes or routines. The goal is to create classes and routines with small, direct, visible, and flexible relations to other classes and routines, which is known as “loose coupling”.

Reference: Wikipedia