We are all intimately familiar with at least one language, yet few of us ever stop to consider what we know about it. The words of a language can be listed in a dictionary, but not all the sentences.
Speakers use a finite set of rules to produce and understand an infinite set of sentences. These rules comprise the grammar of a language. It includes the sound system and the way in which sounds and meanings are related.
This linguistic knowledge is, however, different from linguistic behavior. If you woke up one morning and decided to stop talking, you would still have the knowledge of your language.
Language is a tool of communication. But if language is defined merely as a system of communication, then language is not unique to humans. A basic property of human language is its creative aspect.