A language is a dialect with an army...

The difference between a dialect and a language can be very blurred. In some cases, two dialects can be less intelligible between each other than two distinct languages. Wait, what does that sentence mean? Well here's an example. In Modern English, the separate language of Scots (not to be confused with Scottish English), is more intelligible than the English dialect of Newcastle-Upon-Tyne. This recalls Bell's Seven Criteria, a seven-point criterion that defines the moment a dialect becomes a unique language. When a dialect achieves each of the seven requirements, it becomes a language. With this list, you'll see that the justification of what makes a separate language can be as much political as it is linguistic. (I need to credit the linguist Josh McWhorter for that salient wording, by the way). 

Here's a real example I made. The first example is Scots, the most closely related language to Modern English: 

And this example is Newcastle-Upon-Tyne, a divergent dialect of English from what we consider "typical" London or Received Pronunciation:

Each couple lives only 61 miles from the other couple.