events

1) KeyPress = keydown + keyup;
2) there is nearly no difference between keypress and enter evnets of a button in my mind.
  maybe 'enter' is only keydown event.

The enter event occurs when the button gets focus.  Once the button has focus if you press a key you will activate the keypress event.
You can test it by creating a button with two events like the following porley coded example ;) to see how the various events get handled.
private void testkeypress(object sender, KeyPressEventArgs e)
{
textbox1.Text = "keypress";
}
private void testenter(object sender, EventArgs e)
{
textbox2.Text = "enter";
}