Difference between torque and speed in dc motor?
When the armature of a dc motor rotates , we call it torque because of
  rotational movement about the fixed axis. So where is speed?


We don't call it torque, we call it speed aka rpm or rotational speed.

The force that it takes to spin at a certain speed we call torque and usually, for a dc motor high speed means low torque and high torque means low speed. Look at this formula for the mechanical power output of a motor: -

Power out = \$2\pi n T\$

Where n is speed in revolutions per second or, if you wish \$2\pi n\$ is radians of rotation per second. and T is torque.

For a constant power out n.T must be constant.