Syntax:
EXPLAIN tbl_name

Or:

EXPLAIN [EXTENDED] 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.

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.

URL: http://dev.mysql.com/doc/refman/5.0/en/explain.html

