Step 1: Cocking

Picture
Adam Wainwright of the St. Louis Cardinals
The first movement is to get the arm to the "cocked" position, where the arm is pulled from a neutral position to the position shown here. This movement at the shoulder is called "horizontal abduction." The elbow is bent, or "flexed" the whole time.

Step 2: Horizontal Adduction at the Shoulder

Picture
Stephen Strasburg of the Washington Nationals
The next portion of movement is horizontal adduction, which causes external rotation. This sounds a bit complicated, but it means the muscles act to the elbow moves forward, but keep the hand in place. The elbow is still flexed. 

Step 3: Release

Picture
Mariano Rivera of the New York Yankees
The arm continues to go through horizontal adduction, and the elbow goes from bent to locked out (extension) while rotating over the arm. "Internal rotation" is when the arm goes from the hand being behind the elbow to the hand moving above/ in front of the elbow. 

Step 4: Follow Through

Picture
Hall of Fame Pitcher Nolan Ryan
The arm finishes up with horizontal adduction, while also extending at the shoulder, which means the upper arm moves down, away from the head in front of the body. 
Pictures: Glavine http://cdn.bleacherreport.net/images_root/slides/photos/001/032/268/605804_display_image.jpg?1308609523
Ryan 1 http://news.sportslogos.net/wp-content/uploads/2012/12/Astros-Nolan-Ryan.jpg
Johnson http://assets.espn.go.com/photo/2009/0527/mlb_a_rjohnson2_576.jpg
Bradford  http://www.chrisoleary.com/projects/Baseball/Pitching/Images/Examples/Example_Hyperabduction_AdamWainwright_2006_002.jpg
Wainwright http://www4.pictures.gi.zimbio.com/Florida+Marlins+v+Baltimore+Orioles+AOOd09Zby8yl.jpg
Strasburg  http://ericcressey.com/wp-content/uploads/2010/08/valgus-300x227.jpg
Rivera http://samuraisports.files.wordpress.com/2010/08/mlb_g_rivera_300.jpg
Ryan 2 https://encrypted-tbn1.gstatic.com/images?q=tbn:ANd9GcSZErm3wLuveRbkOZl4YMUKc3ows9etW9UOrEZDAxhlPwhZFl0X