Electric ovens
In a conventional oven the thermostat controls the heat in the middle; the oven will be a little hotter than the set temperature at the top and slightly cooler at the bottom. This zoned heat difference can be useful when cooking different kinds of dishes at once. These ovens also take time to reach cooking temperature, so need to be pre-heated.
Fan ovens
These work differently by using a fan in the rear wall of the oven which has a circular heating element around it. The temperature throughout is even and the oven heats up very quickly, reducing cooking times and so saving energy. With top-end fan ovens you can also cook foods with strong flavours together without fear of these being transferred, because of the circulating air which breaks up odours and fat particles (e.g. with Neff's Circotherm fan system).
However, fan ovens don’t start cooking immediately, so if a recipe calls for a cooking time of less than 25 minutes you should not reduce the cooking time (for cooking times over 25 minutes you should typically deduct 10% off the time).
Multifunction ovens
These give complete flexibility of cooking, using combinations of different heating elements and a fan to present heat to the food. Most multifunction ovens combine a fan and conventional oven together with grill (grilling is done with the door closed, so retaining moisture in the oven and in the food) and fan/grill roasting. The latter produces a finish similar to a spit roast or barbecue.
These ovens may also have other cooking modes such as bottom heat only (for crisping pies, pizza bases etc.) or top heat only (for browning). A rotisserie may also be a feature (the best for meat joints, allowing self-basting), and even a defrost setting which uses the fan and is ideal for meat or even delicate foods such as gateaux.
Gas ovens
Most gas ovens are heated by a flame at the rear of the base, with the gas being ignited by an electric spark. The hot air circulates as it rises, but the top of the oven will always be hotter. Some ovens are shaped to help the air to circulate so that the temperature across the shelves is even. Gas gives off moisture in cooking so it's especially good for cake baking. Some have an economy setting which enable you to use the oven like a slow cooker for casseroles.
Double ovens
These offer great flexibility as well as capacity because you can set each oven differently. Most are built-in with a full-sized multifunction or fan oven below a smaller economy conventional oven with the grill element.
A built-under double oven will still give you flexibility but you will lose some capacity, as more limited space under the worktop means the oven needs to be about 75% of the size of oven built above worktop height.