WordPress lets you associate tags with your posts. Version 2.6 even provides a nice UI to manage them. If you are interested in working with them directly in the database, here is a query to help you along:
SELECT
term.term_id,
tax.count,
term.name,
term.slug
FROM
wp_term_taxonomy tax,
wp_terms term
WHERE
tax.taxonomy = 'post_tag'
AND
tax.term_id = term.term_id
ORDER BY
term.name ASC
LIMIT
0, 200