I have a quick question. Hard to explain, but I'll give it a shot. Imagine you have two rooms, each with outlets in it as per usual. In room 1, you have a GFCI and from it's load side you run the rest of the outlets in the room. From the GFCI line, you go into a junction box, which joins that room with room 2's outlets.

The GFCI should still protect the outlets in room 1 right? And joining other outlets from room 2 at the junction box off the GFCI's line, should be fine right? Basically, anything after the GFCI (load) will be protected, but anything on the line side will not, and should not effect the load side right? Just want to make sure.

And theoretically, you could put one only GFCI outside the junction box, run your circuit to the line, and from load go into the junction box, and therefore protect both rooms, correct?

Your first two sentences make sense. The last sentence I couldn't follow.

Anything connected to the load side is protected. Things connected to the line side are not.

A better way to describe it would be like this, you've got your circuit connected to the GFCI line, and from there you go from the load on the GFCI to the junction box. And from there you wire which ever rooms or outlets to the junction box. Basically I mean, can you protect two rooms with one gfci by connecting the two rooms to the junction box, which comes from the load side of the gfci. I'm having trouble expressing this in words :)

Yep.

Alternatively, You could use a GFCI breaker in the fuse box. There is no real difference.
The primary function of a GFCI is to measure the difference of currents lowing in L1 and N. This subtraction is done directly by a transformer.

It doesn't matter if the outlets are series connected after the GFCI or a combination of series and parallel after the GFCI. They will all be protected.

I was just curious, because I have a room in the basement that I want to protect with a gfci, but I have a couple of other outlets in a different room that goes to the junction box. But they don't really need to be protected. But then I thought, why not protect them all since I'm at it anyway? And since I've got fuses, the gfci outlet is probably my only option right now. At least until I start the upgrade. Thanks again!