Olympic curl bars are designed to reduce stress on the lifter's forearms, wrists and elbows because the angled bar allows a more natural two-handed grip than a straight bar does.
Beginners may find the weight of the bar itself to be a more than sufficient challenge at first. As they progress and gain strength they can add Olympic weight plates, which come in 2.5-lb. increments, to each side of the bar.
Positioning the hands close together or far apart on the curl bar varies the stress exerted on the forearm muscles, allowing a trainer or lifter very fine, specific control of which muscles are being developed; this is why curl bars are reputed to create better muscle development and definition.
This type of bar is--no surprise--used almost exclusively for biceps and triceps exercises, usually curls or extensions. It is not used for power lifts like the clean and jerk or the snatch. Use it to concentrate on fine-tuning your arm strength for sports like rock climbing and skiing.
The word "Olympic" refers to the size of the sleeve on each end of the curl bar, which is sized to fit into a 2-inch diameter hole in an Olympic weight plate. The other common type of bar-and-plate combination has a 1-inch diameter hole in each plate and is known simply as "standard".
Because an Olympic curl bar emphasizes the stress placed on relatively small muscles, always start your lifting program with slightly less than your usual weight for curls to avoid injury.