The term "language" has at least two distinct uses. It can denote the common properties of all systems of human verbal interaction, or more narrowly, individual systems of symbols and the rules by which they are manipulated. We can speak of the human phenomenon of language in general, or of individual languages unique to particular communities of speakers. The scientific study of language, its historical development, characteristics, and use in society is known as Linguistics.
Human language is a naturally occurring phenomenon, and the ability to learn language is instinctive. Though there are thousands of human languages, they all share a number of properties.
While there are thousands of naturally occurring language, humans have also deliberately constructed languages, sometimes called "artificial" languages, such as Esperanto or Klingon intended for interpersonal communication, and programming languages such as Python or Ruby to enable the production of computer systems that can interact with humans. These "artificial" languages are not restricted to the properties shared by natural human languages.
Properties of Naturally Occurring Languages
Languages are more than mere sets of symbols. They contain a grammar, or system of principles, used to manipulate a commonly accepted set of symbols. While this set of symbols alone could be used for expression or communication, it's usefulness would be severly limited without a system for combining the symbols in regular and understandable ways. This is precisely what a grammar provides. It enables a speech community to manipulate its symbols to express clear and regular relationships between them.
The symbols manipulated by a grammar are arbitrary. There is nothing about the English word "door" or the Spanish word "puerta" in themselves, for example, that connects them to the entryway to a room. These words bear a clear relationship to such an entryway, but that relationship is arbitrary. English speakers have assigned one sound sequence to this concept, and Spanish speakers have assigned a completely different one. Any symbol can be mapped onto any concept.
That is not to say, however, that all sound sequences are completely arbitrary. In fact natural languages typically include a small number of words whose sound is intended to mimic the sound produced by something in the natural world. The English word "buzz," for example, imitates the sound that many insects make when they fly. Such onomatopoeic words, however, are not representative of the vast majority of words in any given language.