Like diagonals, curves are more pleasing than ordinary horizontal and vertical lines, and they help to lead the eye into or through the picture.
Curves can be naturally occurring:
And they can be man-made in a natural setting:
Then there are the architectural curves, as in the Portland Head lighthouse:
Just because something, like a lighthouse tower, is round doesn't mean it will photograph as a curve. Seen straight-on, the lines can look straight, so if you're looking for curves in your photo, try taking the picture from an angle.
Spiral staircases make wonderful photos (Snail shells and other spirals look great too.):
Another architectural source of curves is government buildings from the 1950s and early 1960s. This one is in Washington, DC:
Detail shots of old cars have curves too:
Find curves. Photograph them. Try not to get slapped. If you like some of your shots and want to share, post them on your blog and leave a comment here with the link.
Photography 101 - Lesson 1 and Homework
Digital Photography 101 - Quickie Version
Digital Photography 101 - Update
Photography 101 - Lesson 2
Photography 101 - Lesson 3