Every human knows at least one language, spoken or signed. Linguistics is the science of language, including the sounds, words, and grammar rules. Words in languages are finite, but sentences are not. It is this creative aspect of human language that sets it apart from animal languages, which are essentially responses to stimuli.
The rules of a language, also called grammar, are learned as one acquires a language. These rules include phonology, the sound system, morphology, the structure of words, syntax, the combination of words into sentences, semantics, how sounds and meanings are related, and the lexicon, or mental dictionary of words. When you know a language, you know words in that language, i.e. sound units that are related to specific meanings. However, the sounds and meanings of words are arbitrary. For the most part, there is no relationship between the way a word is pronounced (or signed) and its meaning.
Knowing a language encompasses this entire system, but this knowledge (called competence) is different from behavior (called performance.) You may know a language, but you may also choose to not speak it. Although you are not speaking the language, you still know of it. However, if you don't know a language, you cannot speak it at all.
There are two types of grammar: descriptive and prescriptive. Descriptive grammars represent the unconscious knowledge of a language. English speakers, for example, know that "I like apples" is incorrect, and "I like apples" is correct, although the speaker may not be able to explain why. Descriptive grammars do not teach the rules of a language, but rather describe rules that are already known. In contrast, prescriptive grammars dictate what a speaker's grammar should be and they include teaching grammars, which are written to help teach a foreign language.
There are about 7,000 languages in the world right now (a rough estimate), and linguists have discovered that these languages are more alike than different from each other. Some universal concepts and properties are shared by all languages, and these principles are contained in Universal Grammar, which forms the basis of all possible human languages.