Can someone tell me when these two are used and why?
Line voltage is cheap and lame, dude. Low voltage is hip and cool!
Er, Not really.
Low-voltage filaments are short and fat, in comparison to their line-voltage equivalents. The smaller filament means that the light emanates from a smaller "point" in space. Such light is exceptionally crisp and easy to direct with reflectors. It makes diamonds sparkle and shadows more pronounced. A low-voltage reflector bulb can throw a great deal of light in a very specific direction; if you want to throw a spotlight on something and really make it stand out, low-voltage is the way to go.
Low voltage bulbs also tend to be smaller, which is good if you want smaller fixtures. For example, low-voltage undercabinet lights seem to be about 1/2" thinner than similar line-voltage fixtures.
Also, since low-voltage current is less dangerous, it's acceptable to have exposed electrical contacts in places where they might be accidentally touched, which makes some specialty fixtures possible.
Low-voltage requires a transformer to step the current down, which makes the fixtures more expensive. If you want to dim low-voltage lights, you need a special, more expensive, dimmer to do it.
Line voltage filaments, on the other hand, are larger so the light tends to be more diffuse. Shadows are less pronounced. It's a better choice for general lighting, where you don't want hard shadows. The fixtures are cheaper, as are the dimmers to control them.
A few other considerations: as jon1270 explained, low voltage filaments are thicker than line voltage. This makes them more rugged in locations where vibration is a consideration (like slamming cabinet doors).
Line voltage wiring and fixture installation falls under the juristicition of the National Electrical Code and thus must pass inspection. Low voltage does not.
How do you guys know all this (it's an education, that's for sure)?
The NEC covers low voltage lighting systems in article 411. If the wiring system is concealed it has to follow the standard wiring rules in Chapter 3. Class 2 power limited low voltage systems have less restrictions and tend to be easier to install. If you look at a typical "puck" light system such as Hera Lighting you'll notice that they use class 2 transformers. Doorbell systems are another example of class 2.
Having said the above, I've noticed that many inspectors choose to ignore low voltage lighting systems. But it is within their right to inspect it.
Thank you for all the responses. One more question: If I'm not working with a lighting designer (I sure wish now that I was but it is not in the budget!) how do I know when to ask for what from my eletrician? We are building new.
dmlove, I'm addicted to that moment when a mysterious subject starts to make sense. I'm driven to keep reading about certain topics long past the point where a sensible person would've moved on to something more productive.... like doing the laundry. : )
I'm not a lighting designer by no means...but I did stay at a Holiday Inn Express last night.