"Communication" implies two different, and sometimes conflicting, things. On one hand, it means to have a thoughtful exchange of views with a small number of people, perhaps just one. But it can also mean to disseminate broadly a simple message, without deep thought or appeals for feedback. The Latin root is "communicare" and means "to make common."
Communication as a field of study is relatively new. Arguably, it encompasses journalism, public relations, media studies (which might include the study of television, radio, and film), and rhetoric, among other fields.
In a more technical sense, see also telegraphy, telephone, computer network.
We might divide these diverse fields into those which cultivate a thoughtful exchange between a small number of people (debate, talk radio, e-mail, personal letters) on the one hand; and those which disseminate broadly a simple message (public relations, television, Hollywood films.)
Under communication one might also classify the postal system.