Ow, that is a highly debatable topic whose definition varies from person to person. The gist is you avoid processed foods, you eat fresh meats and produce, lean meats or fat-trimmed beef, and whole grains. Avoid all junk foods. In addition, when you cook, you use only the healthy oils (think extra virgin olive oil) in light quantities, or you bake the foods.
This one I (mostly) stick with: link
Should mention, her late husband published a magazine, which contained the deep-level explanations of human physiology and metabolism, which I applied and lost around 90lbs of fat, put on around 15lbs of muscle (estimated by body fat percentage) in little over four months, and I used her plan to eat.