Internal Family Systems, or IFS, is a Freudian-family therapy-descended model of the psyche and psychotherapy method based on the idea that the mind is split between parts called exiles, managers, and firefighters. Therapy consists of methods for allowing these parts to "talk" to each other so, for example, exiles can be reintegrated.
WikipediaBuilding up to an Internal Family Systems model asoffers a more thorough introduction to the IFS model.
Yeah, subagents is the general idea of modeling the mind in terms of independent agents, but IFS is a more specific theory of what kinds of subagents there are. E.g. my sequence has a post about understanding System 1 and System 2 in terms of subagents, while IFS doesn't really have anything to say about that.