The Mishna writes that there is a rabbinical prohibition to eat food cooked by a non-Jew. Rashi explains two possible reasons for this prohibition. The first reason is to prevent friendliness and closeness between Jews and non-Jews, which can lead to intermarriage. This is a similar reason to the prohibition of pat akum. The second reason is because if one eats by a non-Jew all the time, he may end up being served a food that was cooked in a non-kosher pot or contains non-kosher ingredients. Because of the second reason, most poskim agree that even food cooked by a non-religious Jew would be included in this prohibition.
Cooking in regards to bishul akum refers to preparing a food through heat. Included in this is cooking, baking, frying, and steaming. However, foods which were prepared through salting, smoking (cold smoking – for example lox, not hot smoking), or pickling are not included in bishul akum.
 עבודה זרה לה:
 רש”י עבודה זרה לה: ד”ה והשלקות
 רש”י עבודה זרה לח. ד”ה מדרבנן
 פרי מגדים או”ח שכה א”א ס”ק כב, פת”ת יו”ד קיג ס”ק א
 עי’ ש”ך יו”ד סי’ קיב ס”ק ז
 ערוך השולחן יו”ד סי’ קיג סעי’ כד
 שו”ע יו”ד סי’ קיג סעי’ יג