If you speak Latin you can read the Wikipedia explanation here. Seriously though, it is a good, thorough, explanation of every muscle and area in your stomach in Anatomical terms. Below is an explanation of abdominal muscles in plain English.
Your stomach muscles are made up of 4 muscles:
Rectus Abdominis - A long vertical muscle that runs the length of your torso, starting just above your pubic bone and ending in your lower ribs. It provides stability to your torso. The rectus abdominis muscle is the one that, when well defined, gives your stomach a six pack look.
Transversus Abdominis - The deepest-set stomach muscle that runs horizontally underneath the Rectus Abdominis. It contributes to you having a good posture, balance and stability. This muscle wraps around the torso, creating an effect similar to a back support belt.
Stomach Muscles - Obliques
External Oblique - The External Oblique is a broad, thin, muscle that runs in a diagonal line from your ribs to your hips. The external obliques, one on each side of the torso, are involved in, among other things, rotation and lateral flexion of the spine.
Internal Oblique - The Internal Oblique is a broad, thin, muscle beneath the External Oblique that runs at right angle to it. Like the external obliques, the internal obliques, are involved in, among other things, rotation and lateral flexion of the spine.