

Overlock thread is used for finishing, or overlocking, heavily frayed fabrics, as these would otherwise fray into the seam after several wears or washes, resulting in visible holes in the seams. It is thinner than regular sewing thread, yet very tear-resistant.
Overlock yarn is used to finish, also known as linking, fabrics that tend to fray, as these will otherwise fray down to the seam after a few wears or washes and then become visible as holes in the seams. It is thinner than normal sewing thread - and still very tear-resistant.
Because overlock and cover seams need a lot more thread than seams with a household sewing machine, Ovi threads are available in stores on large cones. There is also a reason why the thread itself is finer than a standard general sewer: the seam is worked with up to 4 threads. If the thread were thicker, the seam would be bulky. And that feels uncomfortable when wearing the clothes. And it also doesn't look good, especially with tight-fitting clothing.
Details
Specific References