As you now understand the use and power of dominants, you can relax, you've learned the hard part. 

Now I want you to forget all you learned about "rules."  Because rules are made to be broken.

Secondary Dominants

Let's say you have the following progression.

In the key of C Major: C - Am - Dm - G7 - C

A very simple progression, popular in jazz. 

Now look at this one.

C - A7 - Dm - G7 - C

I've changed the (Am) to an (A7.)  You may remember in tonal harmony, the only dominant seven chord that can exist is the V7 chord.  In normal circumstances, a vi7 chord in C Major would be spelled:

A-C-E-G

But we've changed the C to a C#.  How?

This is a seconday dominant.  And the name really explains it all.  It is a 1) Dominant and 2) Secondary.  A secondary dominant is a dominant acting WITHIN a secondary key.  For two chords you are techniquely in another key.  You are in the key of D minor, the A7 being the V7 and the Dm acting like a (i) chord.  Then we go to G7, back in C Major.

But we don't regard this one measure "key change" an actual key change but refer to it as a secondary dominant.  We could beyond this and change one more thing.

C - A7 - D7 - G7 - C

Here you have the A7 like last time, but we also have D7 too.  D7 is acting as a secondary dominant as well.  The A7 acts like V7 to D.  The D7 then does the same thing but towards G.  The G7 brings us back to the key center of C.  You could do this forever really, also called the circle of fifths.  If you are really interested in chord substitution like this, check out the jazz theory portion of the site.