Syntax:
EXPLAIN tbl_name

Or:

EXPLAIN [EXTENDED | PARTITIONS] SELECT select_options

The EXPLAIN statement can be used either as a synonym for DESCRIBE or
as a way to obtain information about how MySQL executes a SELECT
statement:

o EXPLAIN tbl_name is synonymous with DESCRIBE tbl_name or SHOW COLUMNS
  FROM tbl_name.

  For a description of the DESCRIBE and SHOW COLUMNS statements, see
  [HELP DESCRIBE], and [HELP SHOW COLUMNS].

o When you precede a SELECT statement with the keyword EXPLAIN, MySQL
  displays information from the optimizer about the query execution
  plan. That is, MySQL explains how it would process the SELECT,
  including information about how tables are joined and in which order.
  EXPLAIN EXTENDED can be used to provide additional information.

  For information regarding the use of EXPLAIN and EXPLAIN EXTENDED for
  obtaining query execution plan information, see
  http://dev.mysql.com/doc/refman/5.1/en/using-explain.html.

o EXPLAIN PARTITIONS is available beginning with MySQL 5.1.5. It is
  useful only when examining queries involving partitioned tables.

  For details, see
  http://dev.mysql.com/doc/refman/5.1/en/partitioning-info.html.

URL: http://dev.mysql.com/doc/refman/5.1/en/explain.html

