A hair roller or hair curler is a small tube that is rolled into the hair in order to curl it, making a new hairstyle. Rollers give a longer lasting curl, more volume, and it can be less damaging on the hair than curling Irons.
The size of the roller will determine how much curl or volume you want. The smaller the roller, the tighter the curl. Larger rollers will give you more wave or volume. You want to choose a size according to length of your hair and the desired result.