OOCSS is a way to create stylesheets that are reusable, scalable and easier to maintain. It has two main principles:
Skin & Structure: Structure is how things are laid out, and skin is the styling. These should be separated.
Container & Content: Content refers to images and paragraphs, and containers are the divs in which content lives. Containers should have their own classes, and the contents should not be scoped to their containers.