An array is one of the basic data structures used in computer programming. An array contains a list (or vector) of items such as numeric or string values. Arrays allow programmers to randomly access data. Data can be stored in either one-dimensional or multi-dimensional arrays.

A one-dimension array seven (7) elements would be:

200 54 53 102 13 405

The Template Tag wp_list_categories() uses a one-dimensional array for the 'exclude' parameter.

An example of two-dimensional array, 7 by 3 elements in size, would be:

105 200 54 53 102 13 405
15 210 14 513 2 2313 4512
501 500 499 488 75 1952

Reference http://codex.wordpress.org/Glossary