The SOLID Principle

I've just been reading Uncle Bob's Principles of OOD and came across the SOLID principle. I'd heard this principle mentioned before but had no idea what it was. It's actually an acronym for 5 principles which are:

S - Single Responsibility Principle

O - Open Closed Principle

L - Liskov Substitution Principle

I - Interface Segregation Principle

D - Dependency Inversion Principle