Manual:Interwiki table

The interwiki table stores the interwiki prefixes with their targets.

"DESCRIBE interwiki" confronts us with:

iw_prefix is the prefix of the interwiki link; this is used the same way as a namespace is used when editing.

iw_url is the target of the link; the page name is substituted for $1.

iw_local tells whether the page is local; it is somehow used by the redirect feature. To be perfectly honest, I have no clue how.
 * When, http://en.wikipedia.org/wiki/Meta:Interwiki_table gives a 302 redirect to http://meta.wikimedia.org/wiki/Interwiki_table

iw_trans: is used to allow transcluding templates (or other pages) from another MediaWiki installation. Both  and   need to be on.

"DESCRIBE interwiki" in MediaWiki 1.8 gives the following: +---++--+-+-+---+ +---++--+-+-+---+ +---++--+-+-+---+
 * Field    | Type       | Null | Key | Default | Extra |
 * iw_prefix | char(32)  | NO   | PRI | NULL    |       |
 * iw_url   | char(127)  | NO   |     | NULL    |       |
 * iw_local | tinyint(1) | NO   |     | NULL    |       |
 * iw_trans | tinyint(1) | NO   |     | 0       |       |

mysql> describe mw_interwiki; +---++--+-+-+---+ +---++--+-+-+---+ +---++--+-+-+---+ 4 rows in set (0.00 sec)
 * Field    | Type       | Null | Key | Default | Extra |
 * iw_prefix | char(32)  | NO   | PRI | NULL    |       |
 * iw_url   | char(127)  | NO   |     | NULL    |       |
 * iw_local | tinyint(1) | NO   |     | NULL    |       |
 * iw_trans | tinyint(1) | NO   |     | 0       |       |