Communication is defined as...
...the sharing or exchange of thought by oral, written, or nonverbal means.
The central aspect of communication is "exchange," for without the give and take, the giving and receiving, communication does not take place. Consider trying to speak to someone who does not speak your language or to someone who cannot hear. You can say something, but the other person is unable to "receive" what you are saying, neither the content nor the intent. This refers to an "area of communicative commonality" that is not shared between the two parties. For communication to actually take place...
...the sender has an idea, which he transmits to the receiver through [signs or physical] sensations capable of being perceived by another.
Sometime communication can take place that is nonverbal, through gestures, facial expressions or eye contact (called "body language"). Rolling one's eyes "speaks" volumes, though no words are used. In this case, although words are not exchanged, nonverbal messages are. And so communication still takes place.
A variety of verbal and non-verbal means of communicating exists such as body language, eye contact, sign language...and media such as pictures, graphics, sound, and writing.
Communication can take place, as the quote above relates, through pictorial representations or written words. Morse code, a pattern of sounds or tones (or lights) has been used for many years when only telegraph was available, to cover long visual or spatial distances, and/or to maintain silent communication. Sign language is used to enable those unable to speak to communicate with their hands. Some animals have been taught rudimentary signs to communicate with humans. It is even documented that twins often have a communication system unique to them called idioglossia. In any case...
Language is the predominate method of communication, though it has its setbacks as seen with ambiguity of words spoken, colloquial language, etc. Saying what one means is not always easily achieved if one is not careful to use the correct words.
As Mark Twain once wrote:
The difference between the almost right word & the right word is really a large matter--it's the difference between the lightning bug and the lightning.
Comprehension means everything in communication:
The communication process is complete once the receiver has understood the message of the sender.