Originally Posted by: SnowRoseFirstly, I know they both use the same notes, but in different order, but what makes musicians use D Dorian over C Ionian? [/QUOTE]
It all determines what interval steps are between the notes.

C Ionian
w w h w w w h

Here we have the typical major scale which features these intervals starting at C.
Major 2nd
Major 3rd
Perfect 4th
Perfect 5th
Major 6th
Major 7th
Octave

D Dorian
w h w w w h w

Here we've moved the root note, but since we're hitting the same notes, it alters the intervals between them to this.

Major 2nd
Minor 3rd
Perfect 4th
Perfect 5th
Major 6th
Minor 7th
Octave

Now there are two minor intervals in there. And that's what gives things their own sound. Intervals are the most important part of music since they were what give it the life that it has. Everything else is based off of intervals. All chords are determined by what kind of intervals are in it. All scales are determined by what interval steps are used within an octave.

[QUOTE=SnowRose]
The second question I want to address is:

As for degrees, (C Ionian's 3rd degree is Em), does that make D dorian's 3rd degree Fm or is it still F and why?

It would still be F because the notes have not changed and though the intervals have changed they are still built to accompany the same notes. Since the notes are all the same they're used to make all the same things.

For example
The Eminor chord consists of the notes E, B, G, all of which are present in C Ionian and D Dorian.

In the Fmajor chord, the notes F, C, A are the active notes.

There's a pattern that determins the whole major minor value of notes in a scale. In Ionian it looks like this.
(Legend: M = Major, m = minor, d = diminished)

M m m M M m d

In Dorian it looks like this.

m m M M m d M

So all the chords are still the same, they've just been moved to different positions since the starting position has changed. And that's all a mode really does is just change where you're starting. Have I lost you yet? Let me know if you didn't get something.