At its core, the moving of information. More broadly, the ability to both express oneself effectively through various and appropriate media (writing, speaking, graphics), including understanding audience requirements, needs/pain points, and knowledge, and also effectively listening to others express themselves. Part of communication is knowing what is "in it" for the audience.
All communication has a credibility component.
Communication strategies often need to be created. Having a weekly open house to combat a rampant rumor mill might be critical. As organizations and teams become more distributed, ad hoc (so called "water-cooler") communication becomes less reliable, and so formal communication technique need to be established.
Picking the right genre is important. Letters have different requirements than email. In fact, the best way to trash someone's career is to circulate a colleague's instant-messaging comments reformatted as a letter. Videoconferencing is more formal; web-cams are more casual.
Dynamically customizing the content increases effectiveness. This could be based on audience questions, or for self-paced content, providing alternative paths through the content, such as using hyper-linking.
An effective communication strategy might involve having the audience do some exercises to internalize the information. This could be everything from taking notes at a lecture to playing a marketing mini-game around the release of a new soft drink or movie.
Emotional associations can also be critical. Politicians use the icon of the United States flag. Computer games might tie into famous athletes or movies. Advertisements shamelessly use idealized models, professions, or roles (showing a mother buckling up her children in a minivan while talking about some new fast food product). Create a verbal label for a complicated idea, and you shape the perception of the idea.
There are legal requirements for communications, especially around product recalls and shareholder information. Organizations often have different uses of Public Relations and advertising.
Some communication is passive. What we wear to work or how we style our hair broadcasts our affiliations and even aspirations. Corporations choose colors and fonts carefully.
Some communication strategies are malicious. Spread a lie through so many channels that it becomes thought of as a truth. An ethically-challenged leader leaks a lie to a newspaper anonymously, and then publicly refers to the article as a source of credibility. In the computer world, there are "denial of service" attacks, where many computers try to communicate with a single site in order to overwhelm and shut down the site.
And again, I ask the question that no one will answer. Do you have any formal learning programs around communication? If so, to whom and how? If not, why not: a) it is not important, or b) it is really important, but we don't do it because _____.