The human anatomy

The term "anatomy" is very general, I'll focus on the muscle structure of the body, since this is something I usually couldn't find anywhere until buying an anatomy book.

Besides, note that a few pages of pictures and comments cannot possibly replace the whole book, so I strongly suggest that you buy one. I used a book when drawing these pictures (and I used these pictures when drawing the poses ;)).
I also left out some muscles I considered unnecessary (you don't have to know them unless you're going for extreme detail or studying surgery :)) to make it easier to remember.

The front and back view of the torso. Remember that the torso can be turned and bent, together with the muscles.

The top arm is a back view, the 2 at the bottom are front views. Look at the bottom ones and observe how the arm turns and muscles deform when turning the hand by 180 degrees.

The front and back view of the arm. It's worth a notice that the arm muscles are assymetric.

The front and back view of legs.

The lines on the muscles represent the orientation of muscle tissue. They are usually not visible, though you might want to draw some of them if the muscles are just being used. Otherwise, ignore them, and consider only the whole muscle shapes. Of course, the amount of muscles visible doesn't depend only on their size, but also on the amount of body fat.