Yes. There are rules that cover what constitutes a "house," though. The short version is just a wooden door with certain rules regarding nearby block placement. Each door counts as a separate house, so more doors equals more houses. For example a building with double doors in the front plus a back or side door, counts as 3 "houses" and will support a whole villager (ChaoticSlink is correct in that you need approx. 3 "houses" per villager -- technically they will breed until their population reaches 35% of the number of houses.) They don't have to look like houses, but if they do look like them, then they probably count. See the link in my sig for all the nitty-gritty details.
Funny thing is... I know doors will cause population growth. I built a village on a mountian top. I built one house with a fenced in yard. I had three doors on the house. I moved in two villagers. I built several more houses. None with doors. I went back by the first house and the hearts were flying. No doors added yet. There is something more that will help villager expansion.