An array is a list of values grouped into a single value. An array may be of variable length; in one record the array field may contain two elements, while in the next record, it contains six elements. 

Examples:

["123","456","789"]
["Hello","Goodbye"]
["abc","2"]
["abc","3"]
["A",["B","C"],"D"]

Ragged arrays: The above arrays are collectively a set of ragged arrays. The number of elements in each array varies. When arrays are ragged, some array functions may return null or empty values.

Nested arrays: The last example above is a nested array, in which one array is nested inside of another.