msdnのAbout WindowsのParent or Owner Window Handle
"A window that has a parent is called a child window. "
childでparentがnullの場合はtoplevel

"A window can own, or be owned by, another window."
ここからmsdnのWindow FeaturesのOwned Windowsに移動して
CreateWindowのhwndParentの話になる

要は、child(WS_CHILD)はparentを持ちownerを持たない
WS_POPUPやWS_OVERLAPPEDはparentを持たずownerをもつ
CreateWindowのhwndParentはparentかowner(両義性がある)