The key features of a standard machine needle are called out below. Their configuration varies from needle type to type.
Shank Top of needle that inserts into machine; most often has round front and flat back, which seats needle in right position.
Shaft Body of needle below shank. Shaft thickness determines needle size.
Front groove Slit above needle eye, should be large enough to "cradle" thread for smooth stitches.
Point Needle tip that penetrates fabric to pass thread to bobbin-hook and form stitch. Shape of point varies among needle types.
Scarf Indentation at back of needle. A long scarf helps eliminate skipped stitches by allowing bobbin hook to loop thread more easily. A shorter scarf requires a more perfectly timed machine.
Eye Hole in end of needle through which thread passes. Needle size and type determine size and shape of eye.