OOP

OOP is an approach to programming that developed over time. See http://bitflop.com/articles/procedural-programming-vs-oop.html for a good explanation. If you really want a definition, that can be hard to come by. But you should probably start at http://c2.com/cgi/wiki?DefinitionsForOo

See https://github.com/marcelgsantos/learning-oop-in-php for a fantastic collection of OOP learning resources