APs that are adjacent to one another reside on different controllers.
Salt and pepper is not a recommended design because it can increase inter-controller roaming a lot. The recommendation is to configure what is usually called deterministic redundancy, configure the APs to be strategically placed on controllers.
The idea with salt and pepper is that it can be easier to configure, and supposedly offer better load balancing and redundancy should a controller fall over. You can achieve similar results, as mentioned below, with other designs.
Here is a decent explanation: http://www.cisco.com/c/en/us/td/docs/wireless/technology/controller/deployment/guide/dep.html#wp1060209
