why dc motor called rotating machine

Yoke:- 

A yoke is an outer frame of the DC machine, which is usually made up cast iron / steel. It's help of flow magnetic field line easily through it. Thus, it's give low reluctance path to the magnetic field and it's take half of the main field flux. And, also it's provide the mechanical strength to the DC Machine. 

Pole core:- 

A pole core is a part of DC machine which are also made up of cast steel. The field widing is wound on the pole core due to this when exitiation is appled to the field winding then flux produce. 

Pole shoe:- 

When the magnetic field line produce then pole shoe giving direction to the field line to go straight in the entire machine. As the pole shoe has greater size as compared to pole core thus it's give low reluctance path in which magnetic filed line flow, hence, it's get directions. As AC current in the armature then pole shoe has to be laminated but pole core is not laminated.

A rotating part consist of armature, commutator and brush. But brush are not rotating part it's a stationary part. 

Armature:- 

An armature is a rotating part of an DC machine which are made up of laminated cast iron / steel. An armature core giving housing of the armature winding in which emf induced due to electromagnetic induction. An armature winding are made up mostly copper some time aluminum. Alternative current also induced in the armature winding. 

Commutator:-

A commutator is a mechanical rectifier which reverse the current in the DC machine easily. It's made up hard drawn copper with are insulated from mica insulator. In motor, dc to ac current reversal and in generator, ac to DC by the commutator. 

Brush:-

A brush is a stationary part of DC Machine which taking current form the commutator. And, it's the part which sperate the rotating or stationary part. It is usually made up of carbon.