I can't find something special in motor shield. Why do I need a shield for it? Why not simply supply direct current to the motors (from Arduino board or external)?
Really? Which one are you looking at?
Mine has an L298 motor driver IC, a 4077 CMOS nor gate IC, a bunch of schottky diodes, resistors, capacitors, LEDs. It also has connections for sensors, servos, motors, and an external power supply from which to drive them so I don't burn out my Arduino by running too much current through it.
http://arduino.cc/en/Main/ArduinoMotorShieldR3 has a pretty good description of what it does.