In this video, I cover the differences between octaver, pitch shifter and harmonizer pedals.
In a nutshell:
Octaver: combines your guitar signal with an octave up and/or down.
Pitch shifter: alters the original signal of your guitar for another pitch completely (often done with an expression pedal)
Harmonizer: sounds like multiple guitars playing in harmony (the same thing as the octaver, but you have the choice of way more intervals to harmonize yourself than just octaves - 2nd, 3rd, 4th, 5th, etc).
More in the video above!