S = Single Responsibility
A class should have one responsibility.
O = Open/Closed
A class should be open for extension but closed for modification.
L = Liskov Substitution
Objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program.
I = Interface Segregation
Many client-specific interfaces are better than on general-purpose interface.
D = Dependency Inversion
One should depend upon abstractions rather than concrete classes.