Drawing Arms
On the right is an arm outstretched with the palm facing up. The red lines indicate the tapering of the arm at the elbow, shoulder, and wrist. Also, notice how right at the elbow joint there appears to be a dent.
Looking at the muscle layout below, you can see what makes up the arm and gives it its shape. The green image on the left shows the muscles as basic shapes. There is also a color-coded version dividing the separate muscles (no, our muscles aren't actually colored like this. It'd be cool if they were, though).
Muscle Layout
The red areas indicate joints, bones that make the arm bend. The elbow and wrist joints aren't covered with muscle, but can be diminished by fat. The wrist is made up of only tendons, so this area always remains narrow, no matter how muscular you draw a character.
I am not going into detail on the names because, like bones, you do not need to memorize names, just know where the muscles are placed and how they work together.
Now, how do you start off drawing an arm? Well, there's many ways and everyone has their own preference. What I like to do is draw the joints first, three circles to indicate the wrist, elbow, and shoulder, then connect them with two lines.
Some people start off with shapes first, but it doesn't matter where you start. Do what you are comfortable with.
Right now the arm looks like a stick, so we need to give it some meat. Over the "bones" I've drawn, I draw out the shape of the arm. Notice how I drew the shoulder like it's wrapping itself around the upper arm. That is how the muscles in that area are weaved together.
Once I've got the basic shape done, I start adding in detail, such as the dent in the arm I mentioned earlier, maybe a few tendons showing at the wrist. Unless I am going for a defined, 0%-fat look, I erase some of the line making up the shoulder so it looks like it blends into the upper arm.
On the right are images of the complete lineart for the arm, as well as a shaded version. When drawing using only lines, define muscles and folds with thin, neat strokes. For stuff like paintings, use light and shading.