SOLID is a mnemonic acronym introduced by Michael Feathers for five of the principles named by Robert C. Martin, and when applied together, these intend to make it more likely that a programmer will create a system that is easy to maintain and extend over time.
(++) Single responsibility principle (SRP)
Every class should have a single responsibility, and that responsibility should be entirely encapsulated by the class. All its services should be narrowly aligned with that responsibility.
Open Close Principle (OCP)
Open for extension, closed for modification. Our code should be open to change (or extension) and closed for modification to accomodate that change.
(++) Liskov substitution principle
Objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program.
Interface segregation principle
Many client-specific interfaces are better than one general-purpose interface.
Dependency inversion principle (DIP)
Depend upon abstractions. Do not depend upon concretions. Spring's frameworks popular dependency injection feature is based on this principle.
Note: Latest version of this note can be found @ http://codingarchitect.com/introduction-to-the-solid-principles-of-design.