Hey Boatless,
From the little that I know about fly fishing...
The weight of the fly line you choose is dependent on a few factors...
1) The rating of your rod (which is dependent on the type and size of fish you're after)
2) The distance you wish to achieve
3) The conditions you are fishing
I'm just going to assume that you're planning to fish Wahiawa. I know Stan uses rods anywhere between 4-wt to 6-wt...maybe an 8-wt in heavy cover. So if your rod is weighted 4-wt, you would choose a 4-wt line.
Saying that...the high tech rods today allow you to underweight or overweight the fly line by one number. So if you have a 4-wt rod, you can choose a 3-wt, 4-wt or 5-wt line.
Most people will choose either a 4-wt line for a 4-wt rod...and very rarely a 3-wt line. The lighter 3-wt line will not cast as well on a 4-wt rod.
Some people will choose a 5-wt line for a 4-wt rod. The reason being the heavier line makes it easier to cast. This is especially evident if you are casting on a windy day. The heavier line, in my own experience only, also allows you better control.
Line comes in Forward Weight, Double Taper and Level...and then there are Flaoting, Sink tip/Intermediate or Sinking.
Forward Weight has a fast taper and a thicker section in the head for about 50 feet, then it quickly tapers to a level line for the rest of it. Most people prefer the forward weight because most of the momentum is on the top section of the fly line when you cast it.
The Double Taper is just that. The line tapers the same way on both ends and it is thicker in the middle section. The Double Taper allows you to switch ends of the fly line when one tapered end gets too short after you've retied too many times. Personally, I'd much prefer to use a Weight Forward and buy new line when the time comes than to use a Double Taper...I just find the WF cast better.
Level...I'm not sure what the application is for the level line...perhaps when you are trolling with the fly rod? But there is no taper and no thicker section on a level line. I have used it before...it's okay...but again, I prefer the WF.
Floating line is just that...hollow core fly line that floats. It is used when fishing very shallow with sinking flies or when you are fishing dry flies. If your floating line starts to sink after you've fished it for a bit, it means that your line has some abrasion on the coating allowing the water to soak in. When that happens, it is time for some new line.
The Sink Tip/Intermediate line is used when you need you line to sink a bit when you're fishing streamers, wet flies or nymphs. They have specific sinking rates and only the first few feet of the line sinks. I think the intermediate line will sink progressive so that the line sinks at an angle (but straight instead of forming a belly). But don't quote me on this. I've yet to use this line...but I am planning to try it out in the summer using the line to fish nymphs.
The Sinking line is just that...heavy line with a lead core that sinks. It is most useful for fishing in deep water. Again, they have specific sink rates. I've yet to use this line...I just haven't find a situation where I need it...perhaps this year I'll try it when I take my rod out for pike in the lake.
Most lines (floating, intermediated and sinking) are forward weighted. However, you can also find them in Double Taper or Level. The forward weight does ease casting.
That's all I know about fly lines...I hope I got everything right. I'm sure Stan will make some comments as well...and correct me in some areas