When you say ‘many different kinds’, do you just mean they are made out of different materials? Of course one chooses a rosary that one likes the look (and the feel) of, so different people will have different preferences - wood, or semiprecious stone, or plastic or whatnot.
If you mean they have different numbers of beads, that would be because some of them in fact aren’t Rosaries at all, but beads on which one prays other chaplets.
A chaplet is what is called a ‘rule of prayer’, in other words a structured stringing together of certain numbers of formal prayers. The beads are essentially a mechanism for counting the prayers, and one can just as easily use ones fingers if so inclined.
The Rosary is one type of chaplet. It consists of 5 ‘decades’ (each decade being 10 Hail Marys) interspersed with other prayers.
A St Therese chaplet is another, which consists of 24 Glory Bes, whereas an Infant of Prague chaplet consists of 12 Hail Marys.
So you can tell which chaplet a set of beads can be used for simply by looking at it. A Rosary will have five groups of 10 beads. Unless it’s a single-decade rosary, in which case it will just have 10 beads. A St Therese chaplet will have 24 beads strung together, an Infant of Prague chaplet will have 12.
Another giveaway is that a Rosary will always have a crucifix at the end. Other chaplets tend to have a medal (picture) of whichever saint they are dedicated to in place of the crucifix. Except the Infant of Prague one, it also has a cross but the cross will have an image of the Infant Jesus instead of a crucified body.
