What is the sewing machine that can stitch trim and overcast all in one step?

The machine that can stitch, trim, and overcast all in one step is called a "serger" or "overlock machine".

There are many different brands of sergers. There are several differences that set them apart from a standard sewing machine.

The most notable difference is the number of threads. A serger has 3 or 4 spools of thread across the back, whereas a standard sewing machine has 1. A serger also has no bobbin.

The multiple threads each have their own track through the machine, and they each have their own set of tension disks. The tension disks control the tension of each thread as it runs through the machine. Each tension disk will look like a knob on the front of the machine, resulting in 3 or 4 knobs across the face of the serger.

Multiple threads also require multiple needles. Each thread has a purpose in creating the finished serged seam.

At the base of the machine are also a set of blades. The bottom blade is stationary, and set into the machine. The top blade moves as the sewing machine feeds the fabric, trimming the fabric immediately before it is stitched.

Sergers are often used in garment sewing because a serged seam covers and protects the raw edges of the fabric. This makes the finished garment more durable through multiple washings. Sergers are also preferred when working with knit fabrics, such as t-shirts. The multiple threads in the serged seam make it more flexible and less prone to thread breakage than a seam made with a standard sewing machine. Modern sewing machines often come with the option of a "stretch stitch" which holds up to the stretch of knit fabrics well, but does not create the finished look of a serger.