what is a rosary?

Rosaries are Catholic 'Prayer Beads'. They can be hung on the wall and prayed to, or worn as a necklace or as belt if preferred, and they're said to stimulate the brain while praying as you 'count' the beads and your blessings etc and thank God and all that etc
Jerusalem Rosaries, as i have come to understand although not 100% sure, are normal rosaries but made from a reddish tint to the beads which are 3-8mm wide i think...i think it's just the style and where they came from to be honest. If you're unsure why not ask the person who gave it to you? I'm sure they wouldn't mind.