Ive been trying to figure this out now for a week and its killing me because ive gotten 10 different answers, i know what a sit up is and i know what a pull up is now what exactly is a curl up???
Here is a link to a description of a curl-up:
www.pponline.co.uk/encyc/ab-exercise.html
Also, to work the abs, focus on curling the upperbody. Do not lift the lower back off the floor. Once you lift your lower back off the floor you are working your hip flexors, not the abs. The abs only curl the upper-body – they do not bend it to the legs.
So now I know what to do the next time someone tells me to “curl up” and die. 
thanks i was askin every one but know one knew for sure
Curl ups dont seem to be worth the time. Plenty of ab programs/excersises on this site that would work better. Just a thought.
It’s great that we have Gomez Addams on this site giving advice.