An array is an object that holds values (of any type) not particularly in named properties/keys, but rather in numerically indexed positions:

var arr = [
	"hello world",
	42,
	true
];

arr[0];			// "hello world"
arr[1];			// 42
arr[2];			// true
arr.length;		// 3

typeof arr;		// "object"

The object type refers to a compound value where you can set properties (named locations) that each holds their own values of any type.

var obj = {
	a: "hello world", // property
	b: 42,
	c: true
};

obj.a;		// "hello world", accessed with doted notation
obj.b;		// 42
obj.c;		// true

obj["a"];	// "hello world", accessed with bracket notation
obj["b"];	// 42
obj["c"];	// true

Bracket notation is also useful if you want to access a property/key but the name is stored in another variable, such as:

var obj = {
	a: "hello world",
	b: 42
};

var b = "a";

obj[b];			// "hello world"
obj["b"];		// 42

var string = "Welcome to this Javascript Guide!";

// Output becomes !ediuG tpircsavaJ siht ot emocleW
var reverseEntireSentence = reverseBySeparator(string, "");

// Output becomes emocleW ot siht tpircsavaJ !ediuG
var reverseEachWord = reverseBySeparator(reverseEntireSentence, " ");

function reverseBySeparator(string, separator) {
  return string.split(separator).reverse().join(separator);
}

A callback the function is a function that is passed to another function as an argument and is executed after some operation has been completed. Below is an example of a simple callback function that logs to the console after some operations have been completed.

function modifyArray(arr, callback) {
  // do something to arr here
  arr.push(100);
  // then execute the callback function that was passed
  callback();
}

var arr = [1, 2, 3, 4, 5];

modifyArray(arr, function() {
  console.log("array has been modified", arr);
});