Musculus rectus abdominis (xlas)

Synonyms: abs; rectus abdominis muscle; straight abdominis muscle; six pack; six-pack; sixpack; rectus

Summary

The Musculus rectus abdominis, often referred to as "abs" (short for "abdominal muscles"), sits in front of your stomach and is visible on trained and low body fat people. In fact, there are two individual muscle cords right next to each other (that's why I'll refer to them in plural). Besides being used for breathing the muscles also bend the torso and lift the pelvis forward - thus the abs are the main antagonist of the back extensor muscles (M. erector spinae). There are tendons that separate the long muscle cords into packs for better muscle contraction precision.

Basics

general placement of the M. rectus abdominis

some "good to know" things; the size relations are more important than you think

You can read more about "8 packs" in the advanced section below. 
The 1st row of packs deviates on almost every human being. In general this means, if you draw perfectly aligned abs, they will look less natural.

Origin

• Pubic bone (Os pubis), more specifically on the superior pubic ramus (Ramus superior; ramus = branch)
• Pubic symphysis (Symphysis pubica), thats the cartilage between the two pubic bones
  

Insertion

• Cartilage of the 5th, 6th and 7th rib (there is cartilage at the front tip of each rib)
• Some muscle fibers connect to the breastbone (Sternum)

origin and insertion; some sources do it the other way round, but it doesn't really matter

Functions

• Bending and crunching the torso forward; the familiar "sit up" motion represents this function pretty well
• Lifting/twisting the pelvis forward
• Breathing control; the abs are used mainly for exhaling

How to draw the muscle

this shape helps you drawing the abs; it's a little more emphasized on female bodies

the two main factors are training and body fat percentage; note that women naturally have more subcutaneous fat

Advanced

Reference Pictures (for study only)