Can someone explain EXPLAIN?

Ok, so maybe i'm missing something, but can someone please explain WHAT i'm looking for when i do an EXPLAIN? i mean should it be used as a tool to optimize your query? or just a better understanding of what is going on?

below is my code, that generates a post title, id, author name, date created and category name

mysql> explain select c.id, c.title, s_name, c.created, cat_name
    -> from content as c
    -> inner join cat_relationship as cr on cr.content_id = c.id
    -> inner join category as cat on cr.cat_id = cat.id
    -> inner join user as u on c.user_id = u.id
    -> order by cat_name asc;
+----+-------------+-------+--------+---------------+---------+---------+---------------------------+------+----------------------------------------------+
| id | select_type | table | type   | possible_keys | key     | key_len | ref                       | rows | Extra                                        |
+----+-------------+-------+--------+---------------+---------+---------+---------------------------+------+----------------------------------------------+
|  1 | SIMPLE      | cr    | index  | PRIMARY       | PRIMARY |       8 | NULL                      |    3 | Using index; Using temporary; Using filesort |
|  1 | SIMPLE      | c     | eq_ref | PRIMARY       | PRIMARY |       4 | db20671_cms.cr.content_id |    1 |                                              |
|  1 | SIMPLE      | cat   | eq_ref | PRIMARY       | PRIMARY |       4 | db20671_cms.cr.cat_id     |    1 |                                              |
|  1 | SIMPLE      | u     | eq_ref | PRIMARY       | PRIMARY |       4 | db20671_cms.c.user_id     |    1 |                                              |
+----+-------------+-------+--------+---------------+---------+---------+---------------------------+------+----------------------------------------------+
4 rows in set (0.01 sec)

Code (markup):

 

This may help
http://dev.mysql.com/doc/refman/5.1/en/explain.html

 

it explain your the proccess of query
for example your output tell that mysql first of all selected 3 rows from table cr
than 1 row from c
than 1 row from cat
than 1 row from u