"A sensor is always a device, but a device is not always a sensor. It is important to keep that in mind."
The IoT-lingo can be very confusing: devices are sometimes called sensor, even if there isn't a single sensing function inside, or if it is fact an actuator. While sensors and actuators by definition both are devices, devices on the other hand need not be sensors or actuators.
To set things straight, we've taken the liberty of looking it up in a dictionary for you (source: Merriam-Webster's Online Dictionary):
noun | ac·tu·a·tor |ˈak-chə-ˌwā-tər, -shə-
a device for moving or controlling something.
noun | de·vice | \di-ˈvīs\
an object, machine, or piece of equipment that has been made for some special purpose
noun | sen·sor | ˈsen-ˌsȯr, ˈsen(t)-sər
a device that responds to a physical stimulus (as heat, light, sound, pressure, magnetism, or a particular motion) and transmits a resulting impulse (as for measurement or operating a control)