Human languages are usually referred to as natural languages, and the science of studying them falls under the purview of linguistics. A common progression for natural languages is that they are considered to be first spoken, then written, and then an understanding and explanation of their grammar is attempted.
Languages live, die, move from place to place, and change with time. Any language that ceases to change or develop is categorized as a dead language. Conversely, any language that is in a continuous state of change is known as a living language or modern language.
Making a principled distinction between one language and another is usually impossible. For instance, there are a few dialects of German similar to some dialects of Dutch. The transition between languages within the same language family is sometimes gradual.
Some like to make parallels with biology, where it is not possible to make a well-defined distinction between one species and the next. In either case, the ultimate difficulty may stem from the interactions between languages and populations.
The concepts of Ausbausprache, Abstandsprache and Dachsprache are used to make finer distinctions about the degrees of difference between languages or dialects.