Wordpress' conditional tags are by far one of the most useful and unglorified tools for theme and plugin development. Here's a list of all the tags.
The Conditional Tags can be used in your Template files to change what content is displayed and how that content is displayed on a particular page depending on what conditions that page matches. For example, you might want to display a snippet of text above the series of posts, but only on the main page of your blog. With the is_home() Conditional Tag, that task is made easy.
- is_home()
- is_front_page()
- is_admin()
- is_single()
- is_sticky()
- is_comments_popup()
- comments_open()
- pings_open()
- is_page()
- is_page_template()
- is_category()
- is_archive()
- is_tag()
- has_tag()
- is_author()
- is_date()
- is_year()
- is_month()
- is_day()
- is_time()
- is_search()
- is_404()
- is_paged()
- is_attachment()
- is_singular()
- is_feed()
- is_trackback()
- is_preview()
- has_excerpt()
- in_the_loop()
- is_active_sidebar()
For a description of each visit the WordPress codex.

Nice post but it could have been even better if you'd linked each tag to the right codex page.
Next week, the alphabet :
- A
- B
...
I really very liked this post. Can I copy it to my blog? Thank you in advance. Sincerely
Sure, just put a link back to here in the article.
Hi, thanks,
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hey this is a very interesting article!
I want to learn full PHP very easily, o please send me links. Let me know how to learn PHP very easy way....
bye, thanks
@Raj: You can learn the basics of PHP at W3Schools.
And again! Thank you. Pesky WordPress!