A metronome is simply a devise that provides a steady noise (beep, tap, etc.) to define and keep a musical tempo. The mechanical wind-up type is often seen on a piano and used to keep students on a steady tempo while doing scales or playing a piece of music. A digital metronome requires a battery and uses a digital noise like a blip or a beep to keep the tempo.
Clicktrack
A click track is really just a metronome recorded onto an individual track of a multi-track recorder used to keep all of the individually recorded parts on the same tempo or in sync (synchronization) with each other. Click tracks are used in film scoring to guide the music in speeding up and slowing down as well as hitting the big hits in sync with the visuals. Click tracks are also used in music production especially with electronic music.
Should You Use a Clicktrack?
There is always a debate in the music production world between using a click track or not. One side says that you shouldn’t use a click track because it “sucks the life out of the music” making it “rigid and mechanical”. The other side says that you should us a click track in order to get a tight and consistent sounding track. I have to add my perspective to this argument with two very important insights.
1. Good professional drummers are essentially a click track because of their rock solid and consistent timing, so if you have a human click track there is no need for a 
mechanical one (and that is why a lot of older songs line up pretty closely with a click).
2. As humans, we are drawn to a steady pulse. Musicality is about ebbing and flowing on that pulse.
The click track doesn’t have to suck the life out of music. I have created incredibly expressive music to a stationary click track just by rushing or lagging. It is the fighting against that pulse that pulls on our emotions making performances musical. Even in traditional music (classical music), expressive solos are usually done over a steady pulse with the soloist slowing down and speeding up against it.
The click track is like the law. It can either oppress us or give us freedom. It really depends on how you view it and how you use it.
