I'm putting a 8x16 utility shed in my back yard. It will be 100 ft from the service panel. I want to run the supply line under ground. There will be one 4ft fluorescent light and two 120V receptacles. Which kind of wire would be best for this application?
Why bury a cable when you can be future-proof?
The primary issue with direct buried cables is that you have to dig them up in order to upgrade them, a costly proposition. Hence, it's a far better choice to spend the money to lay a couple of fat PVC conduits now and then pull wires through them, than to have to dig things up 5 years down the road because you want to turn your shed into a shop, park a RV there, or what-have-you, and you need more power for that than the original cable could provide.
Hence, instead of dumping some UF in a trench and calling it good, I would put in two PVC conduits, a 1.5" or 2" for mains wiring and a 1" for low-voltage wiring (such as telephone, CATV, or networking), both buried about 21-22" deep (to provide the Code-required 18" of topcover), and using prefabricated sweeps for the bends up at the ends of the run. This way, you can run 4 10AWG THHNs (2 blacks or black+red, white, and green) for power to the shed for now, with a 100A or 125A, 24 or 30 space, main breaker subpanel at the shed to provide a disconnecting means and room for future expansion, and not ever have to worry about digging up wire and replacing it.
Note that if you go this route, you'll also need to run some 6AWG bare copper wire from the panel to two 8' ground rods, spaced 8' apart, in order to ground the shed properly; this is in addition to having a ground wire in the conduit going back to the main panel. Don't forget to make sure that the bonding screw or strap has been removed from the subpanel, and that the ground and neutral in the subpanel are landed on separate bars, by the way!
Also, you may need expansion fittings on the stub-ups of conduit to take up relative motion between the buried conduit run and the buildings, say due to frost heaving or seismic activity. Check with your AHJ for details. Last but not least, leaving a pull string in the spare conduit is a good idea so that wires can be easily pulled through it in the future.
I'm going to be a bit contrarian and say 10/3 UF. It's pricier and overkill for your existing setup, but here's why it could be worth doing it now.
A single 12/2 means you only get 20 amps at your shed. Period. Next year, you buy some power tools and you start taxing your 20 amp. If the breaker pops, you get to walk back to your main panel. Pop it multiple times in a row and now a thought enters your head
I need more stinkin' power at the shed
Now you get to dig up your 12/2 and re-think your setup. A comment mentioned 12/3, which is better. Now you have 40 amps and 240v, so you should really get a subpanel. This is where 10/3 starts to make more sense. Amazon lists 12/3 UF (100' roll) at $65. 10/3 in the same is $99, or $34 more. 10/3 affords you 30 amps per leg, or 60 amps total. Even if you only use 20 now, you don't need to dig up the 10/3 should you want more power out there. 10/3 UF is commonly available as well, so no trips to an electrical warehouse.
I've built several such sheds, and a #12 UF-B (moisture and UV light resistant) cable (usually gray) is appropriate on a 20A breaker (or smaller). Depending on where you are it may need to be buried to a particular depth. Conduit is a good idea and may reduce the depth requirement. Otherwise, use your best judgement to prevent damage in the future.
Because of the typical usage of small yard sheds like this I don't think cable oversizing or subpanels are needed. Once inside the shed you can convert to regular NM-B cable, which is less expensive and easier to work with. I usually do this at the first outlet or switch box.
If you go small - a single 20A multiwire branch circuit - you don't have to put a subpanel in at the shed. You will still want a disconnect, but they are cheap. At 100', 12/3 UF would do it.
But then again, if you spend a few extra bucks, you could bury 8/3 UF instead of 12/3, and use it for that 20A multiwire branch circuit now, and have an easy upgrade if you ever want to put a subpanel out there and have more power available. Beats digging that trench again.
But if you really want flexibility for the future, you wouldn't use cable.
Since the trench is the most expensive / difficult part of this job, that I don't want to do twice, I'd bury conduit rather than use cable, and have flexibility for the future. If I was going to run conduit, I'd just bite the bullet and run 1-1/4" PVC. For 100' it's not that much more money.
Or you could just run the 12/3 UF. :)
I generally prefer to do property improvements just once in my lifetime. So I'd bury a pair of generously sized schedule 80 pipes below the frostline (using sched 80 instead of normal sched 40 means I can drive heavy vehicles over them - I have a 900+ lb garden tractor), I'd run them in a dead straight line, and I'd put pull strings in them so I could run new wires without digging anything up. Maybe at some point I'd want to put a game camera in there, or an ethernet line.
Since I do all my own work, the biggest obstacle to running wires is principally in the digging. There's lots of roots in my yard! I only want to dig once.
Obviously this is all specific to my circumstances, attitude, budget and environment, but I figured I'd throw it out there. My dad just buried grey UF romex with plenty of slack and it lasted almost 40 years before a suicidal mole or whatever shorted it out. I prefer future-proof installations but if the shed itself won't last more than 40 years, it would make economic sense to do it Dad's way.
Regardless of any other considerations, I'd use armored cable for all the shed wiring because otherwise mice, squirrels and porcupines would eat the insulation off it before long. Can't forget the little red dinguses on the armored cable, either, or seasonal temperature cycling will cause the armor to rub through the wire insulation at the endpoints. All that's actually required by code, here.