"ordinary" DC motor vs DC "servo" motor?

Ordinary brush type DC motors come in several flavors. Series wound motors (also called universal wound because they can run on AC) have the field windings wired in series with the armature, so that the torque produced is proportional to the square of the current (the armature current reacts against the equal field current). Since these produce torque in the same direction, regardless of the direction of current (hence the usability on AC). So these are not usable as servo motors.

Shunt wound motors have the field winding either wired in parallel with the armature, or excited by a separate current, entirely. The separately excited shunt wound motors can be used as servo motors, since their torque is essentially proportional to armature current and their speed is approximately proportional to armature voltage.

Permanent magnet field motors are very similar in character to separately excited wound field motors, since their field's magnetic strength is not related to armature current. They are commonly used as servo motors.

Any motor that can produce torque in either direction can, in theory, be made into a servo motor if you can measure its speed to be used by the servo loop controller. Being able to measure the torque is very handy, also, so motors that have torque proportional to armature current make this easy.

Strictly speaking, a servo is a motor application, not a type of motor.

Ordinary brush motors are just motors starting to turn when a voltage is applied

Stepper motors can be indicated to be servo motors

On the other hand, also ordinary motors may be indicated to be servo motors, depending on their application

Also more generic info at:

As stated above, depending on their application adding an encoder *might* turn an ordinary motor into a servo motor

Another requirement of a servo motor used for precise postioning is it's predictable and consistent response throughout it's operating range. So if it is poorly built it may react to the same voltage and current differently as it spins. This means that the motor may pulse as it spins. It is very hard to control the motor when it acts this way. Modern motors are often built very well because the methods used in production require repeatability. So even cheap motors can often be used as servos if feedback is used. Especially if the application is not very demanding. The servos used for RC models use a potentiometer for position feedback and these devices are inexpensive and robust considering what they cost. ERS