One of the problems with high precision machining operations is play in the machine tool itself. Things like threads are used to advance cutters and such, but ordinary cut thread screws have a certain amount of slop in them and can wear over time resulting in imprecision creeping into the machine. For a lot of run of the mill machining operations, this slop may not be a big deal, but for some things that need to be precise to say 1/10,000th of an inch (ex tiny internal combustion motors like the old Cox .010 cubic inch model engine which isn't much bigger than a $.25 cent piece), slops of any amount make producing the part virtually impossible.
Enter the ball screw. The interposed between the male/female "threads" of the ball screw are small ball bearings -- the male and female ride on those miniature ball bearings and remove virtually all the mechanical slop in the feed screw mechanism and reduce friction during traverse operations.
A wonderful thing the "ball screw idea" is -- much of the high precision small components of today's world would be almost impossible to make without it.
Jose's machining info
The screw itself uses a special spherical thread. That is, it is not made with a pointed angular cutter, but it appears to be made by rolling. The surface is very smooth as you can easily see in the photo[...]