Captions can
- Explain what is going on in the photo
- Identify people whenever possible (especially if their faces are clearly visible)
- Describe the larger context
- Insulting your reader by writing ridiculously simple captions, like "Man in a park with a deer."
- Using gratuitous or superfluous photos or illustrations
- Distorting photos when scaling (hold down that Shift key!)
- Illustrate the story
- Strengthen your layout
- Give your readers a sense of the big picture
(OK, smartypants, if you know so much, add the caption in the comments below …)