An array, like ["foo", 1, "bar", 2] will be decoded into a /list, e.g. list("foo", 1, "bar"), 2. Objects, like {"foo": "bar", "do": "re"}, will be decoded into an associated list, e.g. list("foo" = "bar", "do" = "re"). Additional arrays can be embedded into arrays or objects, and an array can hold multiple objects.

var/my_json = @{"
    {
        "some_key": 1231232,
        "some_other_key": "foo",
        "my_third_key": ["an array", "of strings", 12312, "or whatever"],
        "my_final_key": {
            "another_objects_key": "the other objects value"
        }
    }
"}

var/decoded = json_decode(my_json)
for(var/key in decoded)
world.log << key // some_key, some_other_key, my_third_key, my_final_key

Boolean values, such as false and true, will become 0 and 1 respectively, corresponding to the FALSE and TRUE defines.