What does the "work" part mean in the following English words: network, clockwork, etc?

I know several English words with "work" word as a second part: network, clockwork. Maybe someone can give more examples. They have some meanings in common - they are close to "machinery" or "system", but apart from that they seem very different. What were the historical reasons for producing such words?

The core sense of work is 'anything that's done'. It's not surprising, therefore, that it has many meanings, including those in the words you mention. It's first recorded in the tenth century, and its origins go back much further. It can combine with many other words other in front of or after it. They are too numerous to mention here.