Finishing seams can be important while you are sewing clothing garments. If you sew, you've probably noticed that woven fabrics fray once the fabric is cut. This is not true for knit fabrics, however. If you are sewing a garment with knit fabric (such as a T-shirt), you don't have to do anything to finish the seams, but you can serge them if you would like to.
The first way to finish seams is to serge them. This is done with a machine called a serger, or overlock machine. The standard serger is a 4-thread overlock machine. A serger finishes the edges of fabric by cutting off the frayed bits with a blade and finishing the edge by creating thread loops, so that the edges of the fabric are contained within the loops.
Serging can be done in different ways to finish a seam. The first option is to take your separate pieces of fabric and serge around all the edges first. Then, the garment is put together after all the edges are serged. The second option is to sew the seams together with a regular sewing machine, then, from the inside, serge the two parts of the seam allowance together as one. This helps to add a little extra support for the seam, so it doesn't get ripped while worn. The third option, is to forget the sewing machine all together and just use the serger to put the seams together. This is best for garments with a lot of straight seams that are easy to put together and is the fastest way to finish the seams. The fourth thread (the one farthest left) creates the stitch for the seam, while the other threads finish the edges.
There are other ways to finish seams besides serging. They can also be finished by cutting the edges with pinking shears, which minimizes fraying. You could also apply Fray Check to the edges, which is a glue with keeps the fabric from fraying. You can also sew your garment with French seams, where you sew the seam really close to the edge of the fabric with the wrong sides of the fabric together (so it will be inside out). Then you press the seam open, and then flip it so the right sides are now together, and sew the seam again. Now the edges are encased inside the seam.
Of all the options, serging is the recommended way to finish seams.