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Titles of previous posts found via site search [webmasterworld.com].
my first htaccess file [webmasterworld.com]
.htaccess how big? [webmasterworld.com]
These, and what others will contribute should get you started.
most common uses of the htaccess file are for re-directs etc.
But you can also use htaccess to password protect entire directories. You then use a .htpasswd file with usernames & encrypted passwords in.
This is quite a secure way of password-protecting directories, but will only run on Apache.