Here's a simple way to summarize Western Minor harmony. 

Leaving aside exotic variants, most Western sections in the minor key are constructed from 2 scales and 2 major key modes in varying combinations. 

Lets deal first with the modes:

Consider C Major. The basic baby chord set is:

-------- C, Dm, Em, F, G, Am, and Bo.

figures: I ii iii IV V vi vii o.

The AEOLIAN Mode of C major Starts on A. I'm going to call this A Aeolian.
(But some people call it C Aeolian. Let's not go there today.) 

Note that "A Aeolian" and "A Natural Minor" refer to EXACTLY THE SAME THING.

The basic baby chord set is THE SAME but stars on A:

         Am Bo    C Dm Em  F  G
figures: vi Vii o I ii iii IV V 

(Notice I didn't CHANGE the figures, cos' that WILL drive you crazy.)

Am is the MODAL TONIC. G is the MODAL DOMINANT.

(G, but not necessarily G7. G Sus is more common.)

God Rest Ye Merry Gentlemen is in Aeolian.

You could write a tune with just that.

Now consider G Major. The basic baby chord set is:

         G Am Bm  C  D Em and F#o.
figures: I ii iii IV V vi     vii o.

The DORIAN Mode of G major Starts on A. I'm going to call this A Dorian.
(But some people call it G Dorian. Again, let's not go there today.) 

The basic baby chord set is THE SAME but stars on A:

         Am Bm  C  D Em and F# o  G.
figures: ii iii IV V vi     vii o I .

Am is once again the MODAL TONIC. G is once again the MODAL DOMINANT.

Scarboro Fair is Dorian.

You could write a tune which combines the chords in these 2 modes.

In order to get back to the modal tonic Am you would most likely use the G modal dominant. (Same in both cases).

And that's exactly what everyone did in the middle ages. 

Then along came the Germans. 

The Germans stole an arabic scale from the Spanish and adapted it. Its called the Harmonic Minor:

         Am Bo  C+   Dm E (E7) F and G#o.
figures: i  ii o III+ iv V     VI     vii o .

That fact that the DOMINANT is a nice fat juicy tense E7 gives tunes a more dramtic climax 

E7 -> Am

than the older 

G -> Am resolution.

Now you have three keys with which to weave your progression.

The other Key is A Melodic Minor. (There is rather a lot of controversy and confusion surrounding this that deserves a separate post.)

         Am Bm   C+ D  E (E7) F#o and G#o.
figures: i  ii III+ iv V      vi o     vii o .
 

Now we have four sets of chords to work with.
Very often writers modulate freely between them. 

A realy good example is Greensleeves.  

It turns out that you will frequently find out that 

a section starting out in Aeolian will become Harmonic at the climax
a section starting out in Dorian will become Melodic at the climax,

This is because there is only one note difference between them.

One last point. At any one time the key or mode will be purely Dorian, 
or purely Melodic, or purely Aeolian or purely Harmonic. You won;'t ever see for example F AND F# in the same chord or G AND G#. You can combine these musical structures horizontally but not vertically. (Well you can do what you like, but good luck with your audience!)

See @Matt L 's wonderful Bach example in this post.