Just as the node title says, this is simply a very large clock which is mounted high upon a tower. Usually, these clocktowers also have some sort of audio element, such as an array of bells. The bells then chime every now and then, usually at the top of the hour.
Personally speaking, I fell in love with a clock tower when I visited the University of California at Berkeley. The Sather Tower has to be the most beautiful thing to hear during the day. It's great to hear it ring out while you're working away in your dorm. To date, I haven't had the opportunity to hear a clock tower as grand as the one upon Sather Tower.
If you haven't heard a clocktower yet, whether it be on a church steeple or otherwise, then you are missing out. I wish Squamish had a large clock tower...