Acoustic to Electric...

so is a double stop used with one finger or more than one?

It depends. If I'm fretting -- or, to use the other term, stopping -- two strings at the same fret, I use the same finger. If they're at different frets -- say the seventh and sixth or something like that -- I'll use two fingers.

but then wouldn't that be a power chord? which strings do you strum? both ones you are fretting, or stopping?

Yes. Absolutely. So far as I know, a power chord played on two strings alone could be called a double stop too.

Here's an official definition:
To produce two tones simultaneously on (a stringed instrument) by stopping two strings with one hand while bowing them with the other.
You can find that definition here: http://education.yahoo.com/reference/dictionary/entries/75/d0357550.html

I think the term is originally a violin term, by the way, but I'm not sure. At any rate, the term "power chord" would give you more information (i.e. play the root and the fifth to get this tone) than the term double stop does. So I think we tend to use the term double stop when a powerchord isn't produced. But as I say, a two string power chord is indeed also a double stop.

Oh, and you strum both strings, yes.

Hope that helps.