Why am i getting this php error?

Why am i getting this php error?

$username = $_POST['username'];
$ppassword = $_POST['password'];
$password = md5("$ppassword");
This is the code and i am getting this error
Notice: Undefined index: username in C:\wamp\www\php\compare.php on line 14
Call Stack
# Time Memory Function Location
1 0.0004 250440 {main}( ) ..\lognet.php:0
2 0.0007 260000 include( 'C:\wamp\www\php\compare.php' )
..\lognet.php:3
Notice: Undefined index: password in C:\wamp\www\php\compare.php on line 15
Call Stack
# Time Memory Function Location
1 0.0004 250440 {main}( ) ..\lognet.php:0
2 0.0007 260000 include( 'C:\wamp\www\php\compare.php' )
..\lognet.php:3