Interpersonal and intrapersonal both describe ways of interacting—the first with other people, the second with oneself.
Interpersonal describes relations between people (inter- means “between”). For example, your interpersonal skills determine how easy or hard it is for you to interact with others; and if you engage in interpersonal communication, you communicate with someone.
Intrapersonal is a far less common word. It describes what occurs within a person’s mind or self (intra- means “within”). For example, intrapersonal intelligence is how well you understand yourself; and intrapersonal communication is self-talk, such as when you tell yourself you did a good job.
