This as become a standard that happened kind of organically. It was not planned by a central committee. It represents a subset of Javascript notation:
{
"firstName": "John",
"lastName": "Smith",
"isAlive": true,
"age": 27,
"address": {
"streetAddress": "21 2nd Street",
"city": "New York",
"state": "NY",
"postalCode": "10021-3100"
},
"phoneNumbers": [
{
"type": "home",
"number": "212 555-1234"
},
{
"type": "office",
"number": "646 555-4567"
}
],
"children": ['Mary', 'John', 'Beth'],
"spouse": null
}
JSON is a much simpler standard for data interchange than say XML. Why is that?
Well from Javascript history we know that it was developed by a single designer. Brendan Eich did a good job of the core of Javascript and it was popularized by Douglas Crockford.
Why is JSON good? Well it boils data down to very fundamental structures - simple types - strings and numbers, arrays and dictionaries.
See http://www.json.org/ for a full description of the JSON standard.