According to the dictionary:

At first, I would like to start with some explanations by my dictionary¹:

• Dialect  A form of a language that is spoken in one area with grammar, words and pronunciation that may be different from other forms of the same language. 

• Accent  A way of pronouncing the words of a language that shows which country, area or social class a person comes from.

• Idiolect  The way that a particular person uses language.

• Language  The system of communication in speech and writing that is used by people of a particular country or area.