A "controller" is a power draft device. It's basically a computer controlled fan that stokes or chokes off the air to the fire. With a power draft, you close all the bottom vents but one and use metal foil tape to tape over all of the holes in the open vent except for one. That one vent hole is where the fan adapter mounts. The top vent stays 100% open all the time. What you end up with is there is only one hole in the lower vents open and the air to that is from a fan under computer control. There is a pit temp sensor that the computer uses to see what the temp in the pit is (yeah I know, duh!), and if the temp is below the set point of say 225*, it puffs the fan a little. If it's way below the set temp it turns the fan on full. As it approaches the set temp, the fan stops. The programming in these units does take into account that the pit temp will continue to rise a little after the fan stops. A power draft will usually hold right at the set temp and in a WSM it's like a "set and forget" smoker as it will hold temp as long as there is fuel for the fire to burn.
I've had a 18.5" WSM since 2005. In 2006, I bought a BBQ Guru DigiQ 2 "power draft" or controller. It is hands down the best thing I have ever bought for my WSM. They are a little pricey, but like I said I've had mine 11 years and not a single issue and still using the original probes. You can find other brands and designs that do the same thing for less, but BBQ Guru controllers are a proven product in wide use. It's not a necessity for a smoker, but it sure makes life easier especially on long overnight smokes. I can fill my fire ring with charcoal and let the guru tend the fire all night while I sleep like a baby on an overnight smoke.
The comment about sometimes temps will spike even with a controller is wind can blow in the fan inlet and continue to stoke the fire even with the fire off (not super common, but it can happen).
Oh and one other thing I would add to the above posts about the minion method would be; the temp will spike a little when you put the hot coals on top as the wood chunks light. I generally have my vents choked way down when I'm not using my power draft as I want to keep the pit from spiking early. Also remember anytime you adjust a lower vent on a WSM (not using a power draft), you need to wait at least 5 minutes (and 15 is better) to see what effect your adjustment actually has on the fire. It takes a few minutes for the fire to choke back or stoke up when the airflow is adjusted. If you keep making changes without waiting for it to truly stabilize, you end up chasing highs and low temps like a yo-yo.
This is what my BBQ Guru looks like:
These are not my photos and they are on the 22.5" WSM (with 4 holes in each vent), but it shows how the blower adapter mounts on the WSM.