A standard is a standardized convention - either by a formal specification, or simply because a common convention has gained enough popularity to be dominant.
A de jure standard is a specification published by a standard committee. Some standard committees are ISO, ECMA, DIN, ANSI, and W3C.
Some examples of de jure standards are the A4 paper size (ISO standard 219), the c# language (ECMA-334), etc.
The term 'de jure' is rarely used, and a 'de jure standard' is often just called a standard.
A de facto standard is a custom, convention, product, or system that has achieved a dominant position by public acceptance or market forces"
(source: wikipedia - I could't write it better myself)
A de facto standard does not necessarily follow any formal specification.
As Gudmundur Orn wrote in this answerthis answer, the Microsoft Office DOC format was a de facto standard. It had a dominant position, and it was normally assumed that people could read MS Word documents.
JSON is a funny beast, as it started out as a de facto standard. It has however since been formalized as ECMA-404, so it is now a 'de jure standard'.
It is however also the predominant format for exchanging data with HTTP-based APIs (to my knowledge), thus making it also the 'de facto standard' this purpose.
