A guide to sewing machine needles is available at Sewing.org as of 2015. The guide contains information on general purpose and specialty needles, their typical sizes, the fabrics they are suited to and descriptions of the situations that require each type of needle.
American needle sizes range from 8 to 19, while European sizes range from 60 to 120. Needle sizes are written using both systems; for example, 14/90 or 19/120.
General purpose needles include universal, ball-point and Microtex. Each is available in different sizes and is suited to different jobs; a Microtex needle is fine and sharp, so is better suited to stitching fine fabrics. A ball-point needle is not as sharp but is thicker and stronger, making it better suited to coarse knits.
Specialty needles are used for tasks such as machine embroidery and overlocking, as well as materials such as denim and leather. Embroidery needles are designed to prevent damage to the thread, while leather needles have wedge-shaped points that can drive through leather and suede. Quilting needles are designed to stitch through thick layers of fabric, including seams.
It is important to match the needle to the task; for example, using a leather needle to stitch cotton can damage the fabric.