Just in case you are wondering how post revision works, whenever a post is edited, a new row will be created in wp_posts table. Hence if your posts or pages got edited 10 times, you will have 10 new rows in wp_posts table.

In no time your wp_posts table will be filled up and the post ID will be huge.

To turn off this feature, add this following code to wp-config.php:

define('WP_POST_REVISIONS', false);

You can also delete all post revisions by running this query in phpMyAdmin:

DELETE a,b,c
FROM wp_posts a
LEFT JOIN wp_term_relationships b ON (a.ID = b.object_id)
LEFT JOIN wp_postmeta c ON (a.ID = c.post_id)
WHERE a.post_type = 'revision'

Be sure to backup your database first before performing any queries in phpMyAdmin.

*UPDATE* Auto Saves does not create a revision of the post.

*UPDATE 2* Updated SQL query from Andrei Neculau as the previous query does not delete from wp_postmeta and wp_term_relationships tables.