.htaccess

.htaccess is the file extension. It is not file.htaccess or somepage.htaccess, it is simply named .htaccess

.htaccess files (or “distributed configuration files”) provide a way to make configuration changes on a per-directory basis. A file, containing one or more configuration directives, is placed in a particular document directory, and the directives apply to that directory, and all subdirectories thereof.

htaccess files affect the directory they are placed in and all sub-directories, that is an htaccess file located in your root directory (yoursite.com) would affect yoursite.com/content, yoursite.com/content/contents, etc. It is important to note that this can be prevented (if, for example, you did not want certain htaccess commands to affect a specific directory) by placing a new htaccess file within the directory you don't want affected with certain changes, and removing the specific command(s) from the new htaccess file that you do not want affecting this directory. In short, the nearest htaccess file to the current directory is treated as the htaccess file. If the nearest htaccess file is your global htaccess located in your root, then it affects every single directory in your entire site.

Before you go off and plant htaccess everywhere, read through this and make sure you don't do anything redundant, since it is possible to cause an infinite loop of redirects or errors if you place something weird in the htaccess.

.htaccess informational links

http://www.freewebmasterhelp.com/tutorials/htaccess
http://wsabstract.com/howto/htaccess.shtml
http://httpd.apache.org/docs/2.0/howto/htaccess.html
http://www.buildwebsite4u.com/advanced/htaccess-file.shtml
http://www.clockwatchers.com/htaccess_main.html

404 informational links

Explain what happened:
http://www.nextwaveperformance.com/404

Help them out:
http://www.bioneural.net/404/

Give them a prize:
http://www.cssleak.com/404.php

Give them a way out of trouble with a search box:
http://evcircuit.com/404.php

Give them some options:
http://www.acromediainc.com/404

But not too many:
http://www.microsoft.com/404/

Or too few:
http://www.artlebedev.com/404

Or just be silly:
http://www.coxar.pwp.blueyonder.co.uk/
http://www.galiacho.es/404
http://slonky.com/404
http://suspendedanimations.com/404

Lots more:
http://www.varien.com/blog/turning-a-brick-wall-into-a-speed-bump-26-examples-of-404-pages/
http://css-tricks.com/404-best-practices/