Pass end of URL as $_GET variable to template file

If say your trying to find a variable being passed to it

http://yourdomain.tld/custom/?somevar=weirdo

There are actually several different methods to do this, you can find and validate the $_REQUEST

for example:

if (isset($_REQUEST['somevar'])) $somevar = esc_attr($_REQUEST['somevar']);

Or the WP prefered method where you check if the get_query_var has it.
http://codex.wordpress.org/Function_Reference/get_query_var

I just do this:

if (isset($wp_query->query_vars['somevar'])) $somevar = esc_attr($wp_query->query_vars['somevar']);

Ah you need to create some rewrite rules to handle those

http://codex.wordpress.org/Function_Reference/WP_Rewrite

This allows you to create a new ruleset to use with custom permalinks