Just to add on to leatherback's answer . . .
You seem to be asking about when you see the $row[0] used from fetching database results, right?
There are two kinds of arrays: numeric-based and associative. Numeric based means you access different values in the array using numeric keys. Associative means you access different values in the array using words as the keys. For example:
Associative: $row["field_name"], $var["item1"], $bla["fname"], etc
Numeric: $row["0"], $var["803"], $bla["7"], etc
If you use mysql_fetch_array(), you have access to both an associative array and numeric array.
If you use mysql_fetch_assoc(), you only have access to an associative array.
(Both of those functions are used to fetch results out of a mysql database into an array)
Hope this clears up any confusion.
Cgraz