http://www.vanmar.com.pl/img_meble/dzial_5d/04_regal_5_polkowy.jpg Regał can only have this form - a frame with PÓŁKI built in. Półka is the horizontal board, the one you put things on. It can either be a part of a wardrobe or some other piece of furniture, or a furniture itself: http://www.sosnowemebelki.pl/produkty/polki_wiszace/polka_wiszaca_3.jpg You usually put books or ornaments on regał. Półka - anything.

Okay, thanks for explanation ;)

"Półka" is part of "regał." Półka is a shelf, which is part of the bookstand. You can say "półka z książkami" but it's better to say "półka na książki"