Internal Family Systems (IFS) provides a way of looking at our Beliefs, Emotions, and Behaviors through the lens of internal Parts. This rather complex theory, developed in the 1980s by Richard C Schwartz, posits that "parts" of us are influenced by moments of highly emotionally painful or traumatic events in our childhoods and as we move ahead in our lives, they can become activated, "hijacking" our Present Moment Experience.
Such hijacking is evident when we find ourselves in arguments where we feel like we are fighting for our lives over mundane things (like how to load the dishwasher), or when we tell ourselves that we are Unlovable, or when we feel like our lives are Over because of a breakup and we take to our beds (or to self-injurious behaviors like substance abuse, gambling, impulse shopping, etc).
Gaining an understanding of our Parts and how they influence can be enormously helpful in managing uncomfortable emotions, changing long-held negative beliefs of ourselves, or mitigating harmful behaviors. Here is a brief video that serves as a simple introduction to the theory:
For more information on IFS or to find an IFS therapist see the IFS Institute or the many, many articles, books, and videos on the subject