I welcome corrections from people with more musical theory knowledge than me but I'll give it a try. A song is usually split into two parts, melody and harmony. Melody is the vocal or a monophonic (one note at a time) solo. Harmony is the background to that. A number of notes which fit in with the melody and usually that means chords. Sometimes different instruments play different notes of the harmony. In theory you can add a harmony to a melody in a lot of different ways with lots of different chords which gives a song a radically different feel. Jazz musicians do it all the time.

However, in practice, when a rock or pop musician sits down to write a song they use a keyboard or a guitar and play a set of chords which are usually only 3 or four notes and all the notes in those chords are in the same key. The problem is that playing these already basic chords with a set rhythm is very boring which we know because of all the assholes who strum Wonderwall on acoustic guitar at parties. So to break it up a guitarist for instance will play the chord in different positions on the guitar neck with different inversions. So a chord of C, C-E-G, can be played G-C-E which does sound different. They will also play the notes of the chord individually and in a different order instead of strumming, play part chords or play a riff, a repetetive set of notes which use some of the notes from the chord but with other notes in the same key as that part of the song.

This breaks things up and makes it far more interesting but you can still sit down with a guitar and play the original chords the songwriter used even though their is no guitar part like that on the song.