A switch rod is a style of fly rod. It is a cross between a single-handed fly rod and a two-handed Spey rod. It is a very versatile rod and worth considering for your next rod. You can cast them overhead using one hand or you can use the bottom handle with your other hand to do different Spey casts (think fancy roll casts).
Switch rods are usually between 10 and 12 feet long. They have a top handle about 1-1/2 times longer than a normal single-handed rod and a short 4-inch handle below the reel. This lower handle is used when a two-hand cast is needed.
Manufacturers are now making switch rods in the lighter weights. The lighter #4, #5 and #6 weight rods are most commonly used for trout and smaller steelhead while the heavier #7 and #8 weight switch rods are used for winter steelhead.
You can switch fly fishing methods.
The longer length rod gives you better line control.
They are lighter and less fatiguing than a Spey fly rod.
They are good for tight spots.
They are able to cast heavier flies for distance.
They are good for setting the hook.
Switch rods are larger than a single-hand fly rod of the same weight, so to balance the rod you will need a reel that is about two sizes larger than what a single rod calls for. Another reason is that if you are using Skagit heads and Spey lines they are thicker than standard fly line so a larger capacity reel will be required.
I also would recommend getting an extra spool for your reel. This allows you to swing flies as you move downriver with the Skagit head, then switch to nymph fishing upriver as you work your way back toward your car (or the other way around of course), using the same fly rod.
On one spool I would have a floating running line with a short Skagit head. A set of different Rio MOW sink tips finishes out the setup and can really come in handy when you are swinging streamers or wets.
On the other spool I would have a long belly tapered floating weight forward (WF) line for dead drifting nymphs. I would recommend a line one or two weights heavier than the rod calls for.
If you fly fish in country that often has limited space behind you, you need to consider a switch fly rod. Switch fly rods are a great multipurpose tool allowing you to fish different waters using different methods.
If I were to buy only one fly rod setup for trout, I would get a 5-weight 11-foot switch fly rod. I would have a reel two sizes larger than called for to allow for the thicker Skagit line and have a second spool full of WF floating line for nymph fishing.
Of course I would also have a "leader wallet" full of Rio MOW sink tips.