What on earth is horology? Briefly, it's the science or art of measuring time. It's a science that started back in the days of the Babylonians, who came up with the idea for the 60-second minutes and 60-minute hours we use now.
It's now in the hands of scientists who measure time in billionths of a second with atomic clocks, but most of us aren't interested in billionths of anything. The first timekeeping device was probably a stick in the ground, and some clever soul noticed that the stick's shadow moved as the sun moved. From that came the present sundial. Other devices were the water clock, in which a bowl was either filled or emptied within a certain period of time, probably timed by the shadow of a stick in the ground—but you could use the water clock at night. Other devices were marked candles, oil lamps that would burn a measured amount of oil, or a stick of incense that burned at a measured rate, and would drop a thread-suspended metal ball on a bell.
The first mechanical clock was probably made by a blacksmith somewhere in Italy, as an alarm clock for a brother in a monastery who had to wake his mates at a certain time of the morning to start the day's worship sequence.
From this sprang the monster tower clocks, again made by blacksmiths, that appeared all over Europe after about 1200. These only had one hand because they were so inaccurate—maybe within two hours a day.
With increasing sophistication came the smaller mechanical clocks that the wealthy could keep in their bedrooms, or use as an alarm to wake the servants to prepare Milord and Milady's breakfast. These usually ran for only 30 hours at a winding, and still weren't very accurate. But, life was a bit slower-paced back then; it was kind of hard to buzz off to Cancun for the weekend.
The first watch appeared in about 1500. Not very accurate, but a toy for the wealthy. Over the centuries, with the invention of the hairspring, and other improvements, it became more accurate and smaller until it evolved into the small jewel you wear on your wrist today.
The properties of the pendulum had been known from the time of Galileo, but they weren't applied to the clock until a Dutch mathematician, Christiaan Huygens, used one successfully on a clock in 1673.
Clock accuracy took a spectacular jump—from within hours a day to within minutes a day, and never looked back. Clocks became evermore accurate, designs, both technical and decorative, took off, and hundreds of thousands of people were employed in making clocks, or in making parts for clocks.
Even though technical improvements were being made, further accuracy didn't really come along until Lee Deforest invented the vacuum tube in the 1920s, and the later invention of the transistor in the 1940s.
Take a few minutes to look over the NAWCC horological web listing at Horology - The Index, a massive searchable database of links from all across the web. Whether you're complete novice or a certified expert... if you need horological information on the internet, odds are you can find it here.