In many music traditions, including Western music, the list of all the notes that are expected or allowed in a particular piece of music is a scale. A long tradition of using scales in particular ways has trained listeners to expect certain things from a piece of music. If you hear a song in C major, for example, not only will your ear/brain expect to hear the notes from the C major scale, it will expect to hear them grouped into certain chords (Chords), and it will expect the chords to follow each other in certain patterns (chord progressions (Chords)) and to end in a certain way (a cadence). You don't have to have any musical training at all to have these expectations; you only need to have grown up in a culture that listens to this kind of music.
The expectations for music in a minor key are a little different than for music in a major key. But it is important to notice that you can move that song in C major to E major, G flat major, or any other major key. It will sound basically the same, except that it will sound higher or lower. In the same way, all minor keys are so alike that music can easily be transposed from one minor key to another. (For more on this subject, see Major Scales, Minor Scales, Scales that aren't Major or Minor, and Transposition.)
This sameness is not true for musical traditions that use modes instead of scales. In these traditions, the mode, like a scale, lists the notes that are used in a piece of music. But each mode comes with a different set of expectations in how those notes will be used and arranged.
The above Figure 6.7 shows two scales and two modes. The two major scales use different notes, but the relationship of the notes to each other is very similar. For example, the pattern of half steps and whole steps in each one is the same, and the interval (distance) between the tonic and the dominant is the same. Compare this to the two church modes. The pattern of whole steps and half steps within the octave is different; this would have a major effect on a chant, which would generally stay within the one octave range. Also, the interval between the finalis and the dominant is different, and they are in different places within the Range of the mode. The result is that music in one mode would sound quite different than music in the other mode. You can't simply Transposition: Changing Keys music from one mode to another as you do with scales and keys; modes are too different.