The definition of this term varies by genre. In some genres musicians think of it as a metronome.
In others it’s known basically as an electronic drum loop. Either way it’s main purpose is to keep the band in perfect timing with the song as they play so that they don’t start out the song too fast or slow or change tempos in the middle of the song unconsciously. Also in genres such as gospel music it’s used as a percussive element to enhance the flow of the song and is also most often used as a count in for the song instead of the old school way of the drummer clicking the sticks together. Let’s take a listen to this example that I found on Soundcloud:
Did you hear the electronic drum loop flowing throughout the intro of that song? It also plays through the whole song as well. So in this perspective the click track is made up of often several different percussive elements. Sometimes it may be as simple as a shaker and a finger snap or a very complex drum track with multiple elements and may change up for different parts of the song.
Everything you need to know about creating and running Click Tracks!