Let's take a look at where gas is used in the Japanese house and what the difference between these two types are.
|A Japanese cooking range, or ガスコンロ|
For cooking, the gas will be piped in to a built-in kitchen stovetop (システムキチン) or there will be a hookup for a portable cooking range that you'll buy separately and set on your counter (ガスコンロ).
|An internally installed water heater, or 給湯器|
For water heating, there will be a large unit attached to the outside of your building that provides hot water to all faucets in the house, or the apartment will feature space above, say, your kitchen sink to install a smaller water heater that provides hot water only to that faucet. Both the large and small unit are called 給湯器 (kyuutouki), and the smaller ones will be attached to a hookup not unlike the ones the portable cooking ranges use.
|A gas socket, or ガスコンセント|
Depending on your particular abode, the gas line may also be available for use through a "gas socket" (ガスコンセント, as opposed to an "electrical socket"), a handy little outlet on your wall to which you can connect other stuff, like a gas-dependent space heater.
And if you live in a very new apartment, you might have no gas line whatsoever, because the water heater and stovetop run off electricity. These apartments are often touted in advertisements as being "all electric" or オール電化.
But, let's assume you do have a gas line. And maybe you've already been to the local electronics store to check out appliances. Some are marked as appropriate for LP gas lines, and others are marked as appropriate for natural gas lines. What's the difference?
"Toshi" gas, which means "city gas", is usually called "natural gas" in the English-speaking world. It's the methane-based stuff that is also used for stoves and ovens in the US and UK, albeit with different hookups and more impressively-sized appliances. "Toshi" gas got its name in Japan because usually only large metropolitan areas were outfitted with the infrastructure to get this gas to individual residences.
...Or maybe I should bring that last statement into the present: Rural areas of Japan, and even outskirts of cities, remain without natural gas infrastructure today because of widespread acceptance of its major rival in Japan, LP gas. "LP" stands for liquid petroleum, and this kind of fuel is also commonly known in and out of Japan as propane (for the picky, Japan's LPG is composed primarily of propane with bits of butane and other LPG fuels mixed in).
Now, these two gases are considerably different from each other, which is why appliances have to be designed for one or the other. Not just your appliance, but the hose you run from internal appliances to the gas hookup are also defined by type of gas. If you are using LP, you need an orange hose, which has an anti-flammable rubber design suitable for propane. If you are using toshi, you need a beige hose, which has a durable and heat-resistant rubber composition. Older toshi gas hoses are blue... but if yours is that old, you might want to think about getting a new hose as a safety assurance. Industry standard hoses are rated for 3 years (LP) or 8 years (toshi) of use, by the way.
"Toshi" gas is also rated by how hot it burns. The most common specifications are "12A" and "13A" these days, but other specifications have been widely used in the past, and some locales still use an older specification like "5A." Your local stores will probably only stock devices that meet the local standard, but to be safe, check the specs or ask a salesperson. (LP gas standards are the same nation-wide.)
Phew. We went through a lot of specifics there. Another commonly made comparison among Japanese people, and perhaps more relevant to daily occurrences, is that LP gas is expensive and toshi gas is cheap. Although it is not as huge a difference as I've heard Japanese acquaintances make it out to be, my winter heating bill has differed by a few thousand yen per month depending on the type of gas used in the apartment I was in at the time; if you are in an LP apartment and a cold climate, you might find yourself passing the 10,000 yen per month mark for a daily hot shower.
In fact, if you want to know more about utility costs in Japan, check out this great rundown on the subject from our archives.