An oven is basically a metal box that gets very hot. There are two main types of ovens: gas and electric. A gas oven has an actual flame, usually in the bottom of the oven, that ignites when you turn the oven on. The oven then heats up until the internal thermostat says it has reached the temperature it is set at, it then maintains that temperature. Electric ovens work in the same manner, except instead of a flame, there are electric coils that heat up, often one at both the top and bottom of the oven.
Microwaves are completely different appliances. There is no heat source operating in a microwave. Instead, once you push the start button, your microwave emits, as the name suggests, microwaves. These are waves of energy that penetrate your food, and react with its molecules to get them moving around. This movement, or friction, is what causes the food to heat.
Since they cook in such different ways, you should use your microwave and your oven differently. The more food you put in a microwave, the longer it takes to cook, since the waves have to interact with more food. In an oven, the cooking time will remain the same no matter how much you put in. In addition, because a microwave cooks with no actual flame, your foods won't brown in the same way they do in the oven. So things won't get crispy or dark in the same manner.
Green-conscious users will want to know which is the best choice for their kitchen. The answer is both. Microwaves cook foods quickly, so even though they use a lot of energy, they are good for quick reheating that might take 30 minutes in an oven. But for longer cooking times, stick with the stove.