Well, technically, I think formal dress for a woman is an ankle-length gown, but you see shorter cocktail dresses a lot. And I wear a woman's tux, so I'm really one to be talking.
Almost anything you would wear for a formal evening would not look out of place on semi-formal night. The differences are much more apparent with men's dress. Semi-formal for a woman would be slightly more than business casual, something you would wear on a date or for an evening business event. It doesn't need to look quite as spiffy as formal night. My mother wears things on casual night (that she insists are casual) that would fit right in on formal night, so it's all kinda up in the air.
Semi-formal night is really just another opportunity to get some more pictures of your family all dressed up.
Does that help any? I know it's pretty vague without going into specifics.