The muscles are the organs that enable us to move. They are built up of groups of fibers. The brain sends different commands to the muscles and they contract or extend. The muscles are attached to the bones by connections resembling ropes, called tendons. When the muscles contract they pull the tendons, which in their turn pull the bones and a movement is performed in this way.  
There are over 640 muscles in the human body which do not work independently. They are organized into groups. Different movements work different muscle groups, while the others are resting.

Almost half of our weight is due to the muscles.
 
Voluntary muscles
Most of our movements, for example the movements of the arms and legs, are controlled by our thoughts.
Muscle action is controlled by the brain which sends and receives signals from the nervous system. The muscles which take part in these movements are worked consciously and are called voluntary muscles.

Involuntary muscles
There is also another kind of muscles whose movements are automatically controlled my the brain (the movements of the heart, the diaphragm, the intestines, breathing, etc.). These are called involuntary muscles. It is not necessary for you to think to make these organs work. For instance, the heart beats at the rate of 60 to 80 throbs a second, without us thinking about it.  
The biggest muscle is the sciatic. Its main function is to keep the body erect.
The longest muscle is the hamstring. It begins from the pelvis and reaches the knee. Its length can reach up to half a meter.
The most expressive are the facial muscles. We need 17 muscles to smile and 43 muscles to frown.
The most numerous are the skeletal muscles. When we walk for instance, we use 200 muscles.
The most mobile part of the body is the arm.