A language is a system of communication that uses sounds, symbols and words in order to express a meaning or idea.
A dialect is a regional variety of a language and usually has different words, grammar or pronunciation from the language it is derived from.
Originally, all the languages have once been dialects because they all come from other languages and with time, they start to develop in a different way from the original one by adding words of other languages that are spoken by neighbouring peoples or performing slight differences in pronunciation, etc.
Take latin, for example. At the same time they were conquering vast territories in Europe, Africa and Asia, the Romans imposed their language and made it the lingua franca, that is, the language that everybody had to speak if they wanted to be understood anywhere and by anybody. Most of the peoples they invaded took latin as their own language, but they didn't forget the old language they used to speak, and many of those words remained. Throughout the centuries, latin started to be spoken differently in the many regions that were once part of the Roman Empire. This was in part due to the influence of these pre-latin languages. Probably, in the sixth or seventh centuries a person living in Paris could understand a person living in Rome, but a few centuries later this became much more difficult and eventually impossible.
This means that the original language developed differently in the different regions, forming dialects that much later, when two people from those areas couldn't understand each other, were considered languages.
Another aspect that linguists consider essential about a language is literature. For some reason, some dialects start to develop culturally, and literary works are written in it, while others don't develop this aspect and are consigned to the spoken and not the written form. That's how some dialects become languages and others remain dialects for ever.
In the video below, Mr. Fawlty tries to communicate with Manuel, the Spanish waiter who can't speak English very well, but, unable to make himself understood, he starts to speak Spanish, which is even worse because his Spanish is really poor. He gets confused with Italian when he says “burro” for butter, and he doesn't know that “burro” in Spanish means “donkey”! When his wife remarks that he is supposed to speak Spanish, he replies that his Spanish is classic, while Manuel speaks just a dialect! What a cheek he's got!
Enjoy the video!
You may also like: Learning English can be fun!