There is some really great info about languages over at the Wikipedia Language Portal. It is a great place to start looking into general information on any world language or linguistics and language itself. Furthermore, you can delve into specific detail about many topics in the vast wealth of collective knowledge available at Wikipedia.
A language is a system of symbols, generally known as lexemes, and the rules by which they are manipulated. The word language is also used to refer to the whole phenomenon of language, i.e., the common properties of languages. Though language is commonly used for communication, it is not synonymous with it. The scientific study of language, its historical development, characteristics, and use in society is known as Linguistics.
Human language is a natural phenomenon, and language learning is instinctive in childhood. In their natural form, human languages use patterns of sound or gesture for the symbols in order to communicate with others through the senses. Though there are thousands of human languages, they all share a number of properties from which there are no known deviations.
Humans have also invented (or arguably in some cases discovered) many other languages, including constructed human languages such as Esperanto or Klingon, programming languages such as Python or Ruby, and various mathematical formalisms. These languages are not restricted to the properties shared by natural human languages.
One excellent thing to point out while mentioning Wikipedia is the fact that you can read about a large variety of topics in numerous languages. Just look on the left side for the "In Other Languages" menu, and you can click on any available language and jump over to the corresponding Wikipedia entry - page. portal or what have you. Note: This is not machine translation we are talking about here - it is the current entry for that same topic in other languages - some may be larger, some smaller and many may not exist at all.