Hi, i'll try to answer your question by example:
Since you know ASP (am i right ?), i'm sure you'll agree that PHP is much simpler. Say, i have HTML form like this :
<FORM METHOD="POST" ACTION="uploader.php" ENCTYPE="MULTIPART/FORM_DATA">
<INPUT TYPE="FILE" NAME="thefile">
<INPUT TYPE="SUBMIT" VALUE="Send file !">
</FORM>
then, on file uploader.php I can access uploaded file by referencing on variable $thefile (it is the same as HTML form).
Some of nice things on PHP is you can retrieve various info from uploaded file like followings :
$thefile_name --> returns uploaded file name
$thefile_size --> returns uploaded file size
$thefile_type --> returns uploaded file type, this is much more like MIME-type
To read uploaded file in binary mode, you can consult on PHP manual.
Hope it helps.