Overlocking machines come in various configurations, 3 thread, 4 thread and 5 thread. Choosing between 3, 4 and 5 thread overlocking machines is entirely dependent on your work. They are a great extra to any sewist.
3 thread overlockers will do a simple overlock with no security stitch; this is great if you are going to sew the garment after overlocking.
4 thread overlockers will not only overlock but also do a security stitch which will allow you to complete your task with just the overlocker, an example would be creating a pillow case. 4 thread overlockers are also great for any stretch materials.
5 thread overlockers will do the same stitch as a 4 thread overlock, with the added benefit of an extra security stitch running parallel to the overlocking stitch. Another benefit of a 5 thread overlock is that it can be used as a 3 thread and can also come with a ruffler attachment, allowing you to overlock, sew and ruffle all in one!