Profit: 150 G for Small Milk, 250 G for Medium Milk, 350 G for Large Milk
Care: Try to brush, feed, and milk your cow every day. The more you do this, the better and more valuable milk it will produce. Note that only mature cows can be milked. On a sunny day remember to put your cow outside if you have a field of grass (make sure to build a kind of "fence" around it with wood or rocks so that it doesn't stray too far away from the barn). This will save you money on buying it food and will also increase your cow's happiness.
A cow in this game will eat either Fodder or store bought Cow Feed. Cow Feed costs more overall, but obviously requires less work. To get Fodder, cut it from Grass which you can grow in your fields.