Most entities or objects in most applications can be serialized into a JSON object, with keys and values. A key is the name of a field or property, and a value can be a string, a number, a Boolean, another object, an array of values, or some other specialized type such as a string representing a date or an object representing a geolocation:

{
    "name":         "John Smith",
    "age":          42,
    "confirmed":    true,
    "join_date":    "2014-06-01",
    "home": {
        "lat":      51.5,
        "lon":      0.1
    },
    "accounts": [
        {
            "type": "facebook",
            "id":   "johnsmith"
        },
        {
            "type": "twitter",
            "id":   "johnsmith"
        }
    ]
}

Often, we use the terms object and document interchangeably. However, there is a distinction. An object is just a JSON object—similar to what is known as a hash, hashmap, dictionary, or associative array. Objects may contain other objects. In Elasticsearch, the term document has a specific meaning. It refers to the top-level, or root object that is serialized into JSON and stored in Elasticsearch under a unique ID.