I feel like I'm resurrecting another old thread here, but it may be good information to have out there.
As Indiana Hiker mentioned, there are many types of shifters out there.
There are ATV's with manual clutches, but you find those mostly on the pure sport ATVs. Usually they are two wheel drive, chain driver machines. Actually pretty fun on the trails, but are more geared to racing type riding.
Then you have Auto Clutch shifters. They are pretty much what it sounds like. There is a foot shifter, but the clutch auto engages when you shift gears, all you do is let off the gas to keep it from over revving when shifting. You find these on a lot of mid size Utility, and entry level sport models (250cc to 500cc size engine).
There are also the full automatic ATVs out too. They generally have a shifter you can reach with your left or right hand, depending on the brand. Some just have Forward, Neutral, and Reverse. Others have two forward gears, a High range for normal riding, and Low range for tougher terrains and work duty. They have a CVT belt driven transmission. You just put them in gear and go, no figuring on what gear to put them in. You usually find these on larger Utility ATVs and very few sport ATVs. Originally, back in the 90's, they weren't so reliable, mostly due to water getting on them, but recent years from 2000's and up are much more reliable. The companies got better at sealing the belt up and protecting it from the elements, and are actually very reliable now. You will find these mostly on 450cc ATVs and up, although I have see some new models coming out with them on models down to 300cc.