Well, they're similar in the widest sense, that they're both strategy games, you have to produce resources and fight battles to capture land.

But within the strategy genre, they're actually pretty different. Civilization is turn-based, Widelands/Settlers is real-time strategy. I guess, the latter is at least still relatively slow-paced.
Widelands/Settlers puts a lot more focus on managing supply chains. To produce bread, you'll need a baker, which needs flour and water, and possibly coal, so you need a mill and a farm and a well and a coal mine, and then you need people to actually carry the resources between the buildings, and yeah, it starts to become pretty busy pretty quickly.
If you ask fans of these games, that's kind of what they love the most, that your settlement starts to look like an anthill buzzing with activity in no time.