Both are used. They are practically synonyms for that matter. Πάμε στη βουλή=Let's go to the parliament (building). Το ελληνικό κοινοβούλιο αποφάσισε ότι...=The Greek parliament decided that.... The official title of the Greek Parliament is "Η Βουλή των Ελλήνων" meaning "The will of the Greek people" (βουλή=will, decision), whereas Κοινοβούλιο comes from κοινός(public)+βουλή(will)+ending -ιο. Βουλή is used mostly for the Greek Parliament, but you can refer to the greek parliament as το ελληνικό κοινοβούλιο.
It's true that Βουλή is the word that is used for the building (and not κοινοβούλιο), but is also used for the people and the official title of the greek parliament is Η Βουλή των Ελλήνων. So I would say that both βουλή & κοινοβούλιο are used for the people, but Βουλή is also used for the building.
Jaye16, I just wanted to know if the Greek "κοινοβούλιο" is more similar to the British Parliament or to the American Congress where you have a Senate with 2 Senators from each State (a total of 100) and a House of Representatives which represent each district of each State according to its population (a total of 435 right now). I don't know if Greece is divided in States and has Federal and State governments.