Also listen for
cadences - those moments when the harmonic tension is resolved by changing to a chord that "feels like home".
In TROBB - which I almost spelled Trobb, as if that were a word
- consider 2:08 (Bruce's first vocal after the intro). The first chord there has no harmonic tension, and is the tonic for that section of the song.
Also listen to the how the chords rise (and create tension) as that section continues - and listen for the resolution of that tension when the main riff comes back at 2:16. As you might expect, the cadence at 2:16 signals a return to Em.
Jeffmetal said:
Now, if you're talking about why the chords have their specific names, then it's another story for someone who knows musical theory.
If you mean "why do we call the chords major and minor", it's based on the third of the chord. A major chord contains a major third between root and third, a minor chord contains a minor third in the same place.
"Minor" and "major" are just the terms that have developed over time to represent different types of intervals...
3 half-steps = minor third
4 half-steps = major third
8 half-steps = minor sixth
9 half-steps = major sixth
(There are more, these are just examples.)
In other words, the major interval is always one half-step larger than the minor version.
The scale which contains all the major intervals (2nd, 3rd, 6th, 7th) is the major scale.
The minor scale has all the minor versions, except for the 2nd which is major even in the minor scale.
As to why those particular words were chosen ... that's lost in the dusts of time. Might as well ask why we call the thing you sit on a "chair" instead of a "goozabilly". It's just the word we use.