Remember, you can browse your templates by going to mysite.example/template?browse.

Here is a brief outline of what each template does:

add - displayed when navigating to /anypage?add, used to add within [items] tags
add_feeds - the same as a regular add, except bare URLs are added as feeds
browse - displayed when viewing a file directory, or adding ?browse to any path
chat - the main chat object layout, where a [chat] tag occurs on a page
chat_help - within a chat, this is output when a user types /?
chat_message - this template controls the exact format of all messages within a chat
chat_user - in the users pane, each user will be displayed with this template
chat_whois - this is output when a /whois command is issued in a chat
comments - displayed where a [comments/] tag occurs in a page
create - the layout when entering content to create a page
denied - this is output when the user has insufficient permission to access a regular page
diff - used by the ?diff function, usually spawned from a page history view
edit - the layout when editing content on a regular page
feedstatus - displayed where a [feedstatus] tag occurs in a page
footer - at the bottom of every page
format_guide - when editing or creating a page, this is displayed on the right side
format_guide_forum - when entering a message in a forum, this is displayed on the right side
forum_settings - when viewing /path/to/forum?forum_settings
h - displayed where an [h] tag occurs in a page
header - at the top of every page
history - when viewing a regular page's history
items - displayed where a [items] tag occurs in a page
layout - this is the master template for every page, and controls overall placement of header, body, footer
log - the master change log, displayed when navigating to /anypage?log
login - when logging in
login_ok - after successfully logging in
logout - after successfully logging out
media - this template is referenced on media pages created from /?new, so changes made here will affect all existing media pages
message - used for brief messages, such as when an edit has been completed or a forum message posted OK
new - displayed when navigating to mysite.example/?new
new_media - step 1 for creating a new media page
new_media_2 - step 2 for creating a new media page
new_media_output - the final output template used when creating a new media page
not_found - displayed when a regular page is not found, has links to create a new page, forum, or file directory in-place
pagesel - the page selector used in browse, history, forums
prelayout - a place to set site-wide appearance settings, like text colors, link colors, etc.
protection - the protection settings page, displayed when navigating to /anypage?protection
recommend - displayed where a [recommend/] tag occurs in a page
register - when registering a new user
register_ok - after successfully registering a new user
rules - displayed at the bottom when editing a page or posting a forum message
tags_view - displayed when navigating to mysite.example/?tags=news,local,other
title - at the top of every regular content page
topic_edit - editing a message in a forum
topic_list - listing all topics in a forum
topic_not_found - when someone goes to a non-existant topic URL
topic_post - when posting a new topic
topic_reply - when replying to a topic
topic_view - when viewing a topic
upload - uploading files to a directory
user_levels - displayed when navigating to /anypage?levels