Parliment and its American counterpart act aas a government building. It allows you to vote on certian issues, and to train battalions of units. Since the australian battalions were bugged before, the battalion training feature has since been removed and will be returned in another patch. Typically, each battalion would consist of a number of basic units available to anglo civs (spotooner, militia, rifleman, or light cavalry) and a uu belonging to the faction that you picked at the building (works like consulate). I do not know what issues will be voteable when the feature is added but I can say it is good that they removed it for now because it used to be possible for Australia to send canadian battalions of voltiguers and blackwatch.