Here is a clear description of parts work from an Internal Family Systems perspective. Parts are described as relating to the schemas we develop throughout our lives based on our experiences in life. These are outdated ideas about how we need to live our life because they are based in the past.
There is a difference between therapy that seeks to develop new strategies to overpower the old ones vs therapy that seeks to transform the old strategies. IFS is a type of therapy that can transform these outdated protections and coping methods.