I resisted the urge to make another "history of..." thing and instead found an article that shows all the different kinds of modern christmas lights. I might do more with lights later on simply because I love them.
The Lights You Know and Love
Incandescent lights are the ones that started it all. Even though they're well over a hundred years old now, the technology largely remains the same. The shapes and sizes of the bulbs, on the other hand, have been in constant flux. Now we're left with three major types of incandescent Christmas light bulbs, as described by the excellent guide at JimOnLight.com:
The Mini/Fairy Light: This is the big kahuna. If you haven't seen one of these by now, then you've probably never seen Christmas lights. Traditionally, the set is wired in series, hence the age old problem where if one bulb goes out, the rest won't light. But it's not hard to find sets that are wired in parallel nowadays.
These guys also have a lo-fi twinkle method built in. That little red-tipped bulb that comes with each set is made in a way that as the filament heats up, it rises and breaks the circuit. That, of course, shuts of the rest of the lights. When it cools down, it falls again to complete the circuit, and the lights (wait for it...) come back on. Physics 101.
C7: Again, an incandescent light that comes in a different-sized glass housing. These are about the size of your thumb, and work in almost exactly the same way as a mini light.
C9: You get the picture by now. Same shape as the C7, but slightly bigger.
LED lights have been growing in popularity for the past few years. Regardless of what you think of their light output, there's no denying that they're much more energy efficient than incandescent bulbs, and give off less heat. And who knows, maybe someday they'll match the color temperature of good-ol' tungsten lighting. Until then, here's what you'll be looking at, again according to the guide at JimOnLight.com:
5mm: These are the LED equivalent of incandescent mini-lights. They're small LED bulbs in a plastic enclosure. Usually the "white" level is waaaay off from the "white" of incandescent lights.
G12 and G25: Just like with incandescent lights, you're going to find a whole lot of the same with LEDs, just in different shapes and sizes. These are globe shaped plastic enclosures, G12 is pictured.
C7: You've seen these before, except this time there's an LED inside.
You'll find a bunch of crazy light designs out there, but like Jim's excellent guide says, 99.9% of them are just plastic enclosures that are illuminated by these types of bulbs.