Human languages are usually referred to as natural languages, and the science of studying them is linguistics. A common progression for natural languages is that they are first spoken, then written, and then an understanding and explanation of their grammar (according to speech) is attempted.

Languages live, die, move from place to place, and change with time. Any language that stops changing begins to die citation needed; any language that is a living language is in a state of continuous change.

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 (see dialect continuum).

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. (See Dialect or August Schleicher for a longer discussion.)

The concepts of Ausbausprache, Abstandsprache and Dachsprache are used to make finer distinctions about the degrees of difference between languages or dialects.