In psychology, the term "interpersonal" refers to what happens between two people. For instance, interpersonal communication would refer to the things an individual says to his friend, parent, coworker, and so on—as well as the things they say back to him. An interpersonal relationship is a connection between two people—the emotional, physical, and social bonds they share.
On the other hand, "intrapersonal" refers to what happens cognitively and emotionally inside an individual. Intrapersonal communication would be the self-talk that an individual engages in (e.g., "I can do this!," "This isn't so bad," or "I'm not good enough. I'll never get this right," etc.). Likewise, an intrapersonal relationship would consist of the connections of an individual to his/her own thoughts, feelings, and identity. This has a lot to do with an individual's concept of self, as well as their perceptions and beliefs about the world around them.