As photographers, we generally strive for level images; a tilted horizon is one of the first things we notice in a photo. However, what looks level and what's actually level might be two different things, and this great video examines the difference.
Coming to you from Ben Horne, this video examines a subtle issue, namely what "level" really is in a photo. When we're out in nature, we have additional clues to tell our eyes what level is: the context of the surrounding scenery, our inner ears, and the pull of gravity in particular. However, once we're back at our computer and looking at an image, all those clues are no longer present, and an image of naturally sloped scenery may suddenly look weird and askew. This is when you might want to consider making a distinction between visually level and truly level. It won't always be the case that you'll want to make the image visually level; sometimes, it'll have its own visual cues to indicate the correct angle (think of houses rising vertically against a sharply angled San Francisco street). Nonetheless, it's something worth considering when the image looks a bit strange even though you know it was level when you shot it.