What's the difference between 包丁[ほうちょう] and ナイフ?
When one word if preferred over the other?
包丁 should be a larger knife which is used for cutting vegetables or meats. So, it is used to prepare large meals. (especially suitable for eastern Asian cuisine)
ナイフ should be a smaller knife which is used for cutting fruits or cakes. So, it is used to prepare desserts or something not so large. (especially suitable for western cuisine)
In addition to food preparation, ナイフ is used to describe a table knife (e.g. for eating a steak) or an army knife (e.g. to protect yourself). We do not use 包丁 in these situations.