I'll elaborate a bit more on ChrisR awesome answer and bring images from his awesome reference.

A valid JSON always starts with either curly braces { or square brackets [, nothing else.

{ will start an object:

{ "key": value, "another key": value }

Hint: although javascript accepts single quotes ', JSON only takes double ones ".

[ will start an array:

[value, value]

Hint: spaces among elements are always ignored by any JSON parser.

And value is an object, array, string, number, bool or null:

So yeah, ["a", "b"] is a perfectly valid JSON, like you could try on the link Manish pointed.

Here are a few extra valid JSON examples, one per block:

{}

[0]

{"__comment": "json doesn't accept comments and you should not be commenting even in this way", "avoid!": "also, never add more than one key per line, like this"}

[{   "why":null} ]

{
  "not true": [0, false],
  "true": true,
  "not null": [0, 1, false, true, {
    "obj": null
  }, "a string"]
}

The JSON for that array is:

["apple","orange","cat","dog"]

JSON for arrays is the same as Javascript array literals, except that JSON doesn't allow missing elements or an optional trailing comma. And the elements of the array have to be valid JSON, so you have to use double quotes around strings, you can't use single quotes like Javascript allows.

You generally shouldn't have to worry about how to format JSON yourself, most languages have library functions that do it for you. In JS you use JSON.stringify, in PHP you use json_encode().