In Ye olden days DC generators were brushed commutated devices. They had a one or more stator windings and an armature winding. Field wound DC generators as well as motors were commonly connected in one of three methods: Series, Shunt and Compound. Without getting into details, each had its own set of strengths and weaknesses. But you only have to remember these two things: the voltage of a DC motor is dependent on its input shaft speed. Current is a function of torque. More voltage means more RPM's and more amps means more newton-meters (or foot-pounds).
So with all that, you need a constant speed source to get a constant voltage. And you need to ensure you have enough torque to satisfy the current demand of your load otherwise voltage drops off. Old automobiles had commutated generators. They couldn't regulate the voltage so they used a range of around 10-14 volts and used a relay that simply closed when the engines speed was within the voltage range. If the voltage went too low or too high, the relay opened. Primitive by today's standards. The Alternator in today's automobile uses a voltage regulation circuit that varies the armature current which changes the field strength based on the stators output voltage. Lower speed means more current to the armature and less current at higher speeds.
So how different were DC generators from motors? Not very different at all. If anything they mostly differed in mechanical design as they were to be coupled to a prime mover (steam, ICE, electric etc.). Though, in much larger dynamos they had adjustable commutator brushes to compensate for the shift in the commutation plane as a result of heavy load characteristics. A hand wheel would turn a worm gear which would advance or retard the commutation plane to bring the generator back into its normal operating parameters. You don't need to worry about this as I am sure you motor isn't megawatt capable.
I am guessing your motor is a permanent magnet type motor. Its nameplate RPM is what you need to spin the motor at to get the nameplate voltage. This means if you have a 12V motor that spins at 6000 RPM, you need 6000 RPM to get 12V. If you don't have a constant speed source you have no way to regulate the voltage. You would need a buck-boost switching regulator to get a constant voltage from your motor.
If you are using this for a renewable energy project like wind or hydro, a charge controller is usually designed for a wide input voltage swing via a buck/boost regulator. Solar panels are a close analogy to a permanent magnet DC generator, no internal voltage regulation and a varying amount if input energy. Sun might be shining bright one minute and a minute late, be blocked by a cloud. So the charge controller does its best to make a useful steady-voltage from its varying input. From there, use storage batteries to capture that power for later use and to act as a buffer for low input events.
And just for reference, an AC motor can also generate power if you spin it faster than its nameplate RPM, usually at synchronous speed. But again, no voltage regulation and a constant speed is needed. More trouble than its worth. Also of note: jet planes use a very elaborate mechanical speed regulator to produce constant shaft speeds which ensures a constant 60 or 400Hz AC frequency as the throttle is varied.