The following comments and content courtesy of peregrines.
What are the different fly rods used for?
Advice sometimes tends to be all over the place based on the different types of fishing people do in their local waters. Let us know where you are located, and give a brief description of the waters you fish (lily pads, streams, big rivers, deep lakes) and what other types of fishing you might want to do down the road (FW trout? saltwater redfish etc).
So what 's the difference? Here's sort of a break down of fly rod weights (remember "weight" refers to the amount of weight of line the rod is actually designed to cast, not the actual weight of the rod.) There is a lot of overlap, and there is nothing written in stone about what weights you can or can't use, but in general this is sort of how it shakes out:
1, 2, 3, 4 light rod weights for casting small flies, generally for small fish, mostly panfish and trout. These rods are limited in their casting distance and size of flies, and I would not recommend one for your first fly rod. They are just not as versatile as the heavier weights in terms of their ability to cast a wider size range of flies, deal with unfavorable conditions (wind), and all things being equal, your casting distance will be more limited. With the size and wind resistance of flies, poppers commonly used in bass fishing, you'll be even more limited.
5 weight- probably the most popular all around trout weight, though 4 and 6 weights are also commonly used. Will handle small 10-12 panfish sized panfish poppers ok but will be more of a struggle to throw the larger bass stuff, especially if you need to cover a lot of water. A 5 might be a good choice if you planned on doing a lot of trout fishing and chase small bass on small farm ponds.
6 weight a good trout weight especially for large rivers, and has more oomph for casting occasional bass sized poppers, medium sized streamers, and weighted streamers like clousers and buggers and small medium sized bass flies. A very versatile rod weight for small bass, and occasional trout and panfish.
7 intermediate between 6 and 8, a bit better for casting larger flies and poppers for bass, but a bit light for SW. A good smallie rod for throwing clousers and crayfish imitations, but I wouldn't go lighter than an 8 if you're thinking of also using this rod in saltwater down the road.
8 for casting larger bass poppers, deer hair bass bugs and fishing for bass in heavy cover. Also an excellent weight for steelhead, inland striped bass and occasional pike and light salt water (redfish, bonefish, snook, speckled trout, school size striped, bass bluefish).
9-10 Heavy freshwater salmon pike musky, inshore salt permit, striped bass, bluefish, baby-medium tarpon, light offshore ( cobia, mahi).
11-12 Heavy fish fighting sticks for large tarpon, small tuna, big sharks
13 bluewater tuna, sailfish. marlin Very specialized rods built more for fish fighting than casting.
As far as buying an outfit, there are a lot of very good choices out there. Sage Launch is very good, but there are also outfits from LL Bean around 100 bucks or so (rod reel and line), and rods like the TFO Pro for about 150. Your fly line should be matched to the rod with a good quality weight forward floating line from Scientific Anglers or Rio 35-60 bucks. An inexpensive reel is all you need for bass and most other FW fishing. (For SW and steelhead you might want to spend a bit more for a higher quality drag).
Hope this helps. Let us know more about the waters you plan to fish and keep asking questions!
In short, to produce profit for the shops/brands that sell/make the rods.
There's a lot of overkill in the industry attempting to muddy the info needed by the uninformed customer to a wonderfully enjoyable outdoor activity.
Location: White City (tad north of Medford) Oar-E-Gone
Re: What are the different fly rods used for?
I can see the 'I'll just sit this one out posts above.'
Most of my fishing is done with a 2hander so take this with a grain of salt. But as for 1handers I've found that a 3, a 5, and a 7 cover 95% of any use I'd have for them. Do have an '8' but its been years since that was taken down off the garage wall.