Answer by Marc Ettlinger, Ph.D., linguistics, UC–Berkeley:
The pioneering sociolinguist and Yiddish scholar Max Weinreich had a quote:*
אַ שפּראַך איז אַ דיאַלעקט מיט אַן אַרמיי און פֿלאָט
a shprach eez a deealekt mit an armee un flot
A language is a dialect with an army and navy.
His point being that the difference between a language and a dialect was ultimately a political distinction and had little to do with linguistics per se. Thus, German and Dutch are separate languages, but Mandarin and Meixian Chinese are supposed dialects.
Linguists, however, do make a distinction between the two based on the concept of. Two languages where speakers can understand each other are considered dialects of the same language, whereas two languages where the speakers cannot understand each other are, indeed, separate languages.
This often isn't black and white, since understand can be a hard criterion to pin down. There's no well-established way this has been operationalized.
There is also the notion of a, wherein languages A and B are mutually intelligible and languages B and C are mutually intelligible, but A and C are not. This is very characteristic of the Bantu languages in Africa, for exampe. So which are the languages and what are the dialects in this case? It's not completely clear.
Historically, when two dialects are in close enough continuous contact with each other, they will often remain mutually intelligible. With enough separation in time and space, though, dialects will eventually turn into separate languages as the two become more and more distinct.
More questions on Linguistics:
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- Is a pure phonetic orthography desirable, at least for English?
Correction, Feb. 6, 2014: This post originally misattributed a quote by Max Weinreich to his son Uriel Weinreich.