Marriage in Harvest Moon.
To start the process, you first need to get the bachelorette of your choice to a little more than five hearts. You can do this by talking to them and giving them gifts, and if you so please, if they ask you a question, enter a building (or exit, if your inside), go back, and talk to her again, repeat this enough times and you'll blow through this step. Once you have the bachelorette at a little more than five hearts plus the first house upgrade, you can purchase a blue feather from the peddler on Sunday and give it to your soon to be fiance on Saturday (Sunday if going for Nina).
After the wedding, your wife will change into a rather bland lavender dress and move in with you. Each wife will do a different thing once married. Nina will do laundry, Ann will make inventions, etc. etc. If you go to bed too early or come home too late, your affection points with your wife will go down (if your wife is Eve, it won't matter). The best way to counter-act this is to give your wife an egg everyday, since their likes and dislikes change post-marriage, and eggs just so happens to be something they all want. Their role in your farm life? Nothing really, they only do fake chores like the previously mentioned laundry and inventing, and cook a dinner for you to refill your stamina, which is useful when you get to the mega farm phase.
For more information about the bachelorettes, see the characters section.