Communication is simply the act of transferring information from one place, person or group to another. Every communication involves (at least) one sender, a message and a recipient. The transmission of the message from sender to recipient can be affected by a huge range of things.
Communication theory is a field of information theory and mathematics that studies the technical process of information, as well as a field of psychology, sociology, semiotics and anthropology studying interpersonal communication and intrapersonal communication.