file upload troubles

Hi ! I'm having terrible troubles with the file upload program I need. I have read a couple of tutorials and studied the examples in the manual, but it won't work. Every tutorial I read worked with variables $usrefile, $userfile_name etc. When I copy the same example code , I get the error msg. that $usefile_name is unkown. I'd also like to know what does enctype=\"multipart/form-data\" mean and what is its function. Thanks Nikola