Well, most sailors use the two terms interchangeably, but in general:
A Jib is 100% of the foretriangle (The triangle made between the forestay, mast and deck). A genoa is anything past that. A 155 Genoa, for example, would be 155% of the foretriangle.
As far as reefing with a roller furling, this should be your basic technique (at least as I do it).
1) Build up forward speed.
2) Grab the furling line (ususally rigged to port, in my experience) and hold the tiller or wheel in your right.
3) Steer almost (ALMOST) dead into the wind and pull it in quickly.
4) Cleat off the furling line and fall back off to regain momentum.
Most people (myself included) will mark their reefing line with different colors so that you can know at a glance what % you are at: 135, 100, 75, etc. These colors correspond with where they would fall on the cleat on my boat... but whatever works for you... as long as it is the same for all the colors.
Just so you know, a jib is a much better sail for heavy air than a jenny, because after you reef in a genoa past about 75% or so (in my opinion) it has lost so much of its sail shape it does not perform well at all. This is why many boats have a inner forestay so they can run a smaller storm jib or working jib while still maintaining their Genoa for light airs.
Hope all that made sense. You will get the hang of it after you do it a few times. Don't get discouraged. It is like riding a bike: you have to fall a few times before you get it down.
Write back with any questions. Have fun with the boat.