The earliest clocks were simply ways of keeping track of time using the elements. The sundial did this, as well as a clock created in ancient Greece often ascribed to Plato, that used water dripping from a bowl. The Romans later took this clock and claimed it for their own, calling it a Horologium.
The first mechanical clocks were run using the falling of weights. When they started to become mechanical, they were called Clockes, Latin for bell. This was because they kept time by sounding a bell at the hour. When they invented a clock that could tell time visually, they named it a watch. This is an obvious pun on the word watch, both in its to see meaning, and in its meaning for sentries such as in Ill take first watch. In the early days when a watch was made that also chimed, it was called as clock-watch.
The next big improvement in the clock was the creation of what is known as average escapement. This was a device that allowed equal distribution of the force in a clock, allowing them to keep time accurately. Most early clocks were large and expensive, due to the precision hand crafting that went into them. It wasnt until the seventeenth century that clocks started to be seen in homes, mainly in Britain.