Tachometer vs. Tachogenerator
Tachometer is an instrument to display RPM of some rotating shaft.

A tachometer consists of tachogenerator (a sensor on the rotating shaft), and an electronic circuit that displays the sensed input. 

The tachogenerator might generate either analog voltage proportional to RPM ("analog tachos"), or square pulses with period proportional to RPM. Or it can be a unit that already sends its output in some digital serial format.

The display could be an analog dial, or alphanumerical digital, or stepped motor-based, or some digital output to feed an on-board computer. 

A tachogenerator can be a part of control loop, while tachometer is just a display for human readings.