for starters...a 2 stroke refers to the # of strokes that the piston makes, to complete the combustion process. A 2 stroke engine's piston makes 2 strokes...down (intake) and then back up (exhaust). there are intake and exhaust ports that are covered by the piston during the process so the engine can make power using only 2 strokes of the piston. a 2 stroke does not have actual valves, but they can use reed valves to make more power. (like the reeds in a woodwind instrument). the majority of 2 stroke engines for bikes do not use reed valves, but they have ports that are covered and uncovered by the piston as it moves up & down (an intake port and an exhaust port.) 2 stroke engines do not have a supply of oil in the crank case to lubricate the bearings and piston rings/ cylinder wall. the oil for lubrication is in the gas (that's why 2 strokes require a mix of gas and 2 stroke oil). a 2 stroke will generally rev a lot higher than a 4 stroke, and in some instances 2 strokes will make a lot more power than a 4 stroke will, especially at higher rpms.
a 4 stroke is a lot different...the piston makes 4 strokes to complete the combustion process. intake stroke (piston goes down, intake valve opens and fuel/air is sucked into the cylinder...intake valve closes), compression stroke(piston goes back up and compresses the fuel/air mixture. both the intake valve and exhaust valve are closed), power stroke (while the piston almost to the very top of the cylinder during the compression stroke, the spark plug fires and ignites the compressed air/fuel mix. the explosion pushes the piston back down, causing it to make power and turn the crankshaft). exhaust stroke (the piston goes back up, and the exhaust valve opens. the rising piston pushes the exhaust gasses out through the exhaust valve and out of the cylinder). and then the process starts all over again.
4 strokes use intake and exhaust valves just like a car engine, they have a cam shaft that actuates tappets (or lifters) that open and close the valves at a precise time matched to the position of the piston. a 4 stroke must have a supply of oil in the crankcase (just like a car engine) to lubricate the bearings and the piston rings/ cylinder walls. 4 strokes do not rev as high as 2 strokes do, but a 4 stroke can make a lot more power than a 2 stroke at low rpms. a 4 stroke only uses gasoline..no gas/oil mix.
think of it this way. think 4 stroke (car engine, lawnmower engine, lawn tractor engine, street motorcycle engine, snowblower engine and now newer dirt bike engines) 2 stroke ( weedeater, chain saw, leaf blower, outboard boat engine, older dirt bike engines, snowmobile engine)
2 strokes have a very high, raspy exhaust sound, where a 4 stroke has a deeper, thumpier exhaust sound.
a 2 stroke engine (depending on the build quality) can last a VERY long time if maintained and the correct fuel /oil ratio is used, there are less moving parts in a 2 stroke than in a 4 stroke...less parts that can break or wear out. a 4 stroke engine can last a very long time as well for the same reasons, but there are a lot more parts that could potentially break or fail due to age. run a 4 stroke 1 quart low on oil, and you'll toast the crankshaft bearings.
on the other hand, run a 2 stroke on just straight gasoline, with no 2 stroke oil added and the engine will lock up from lack of lubrication.
there is a lot more tinkering and tuning, playing with air fuel ratio's and fuel / oil ratios with a 2 stroke v.s. a 4 stroke, but both types of engines do require some fine tuning at one point or another.
is one better than the other? it's all a matter of personal preferance i guess. a lot of guys like 2 strokes because of the amazing amount of power that you can get out of a very small engine, by tuning and adding aftermarket exhaust pipes & stuff like that.
other guys like 4 strokes because they are basically "plug & play" and only require general maintanance. altho you can add aftermarket parts to make a 4 stroke make more power.
but generally speaking, since 4 strokes operate best at lower rpms, they make plenty of power when it's needed. 2 strokes usually have to rev very high to make big power.
the thign is, you should really do some research on both engines. if you are going to build a motorized bike, do the research and get as many opinions as you can about both engines before you make a decision on which one you want to buy.
generally, 2 strokes are cheaper to buy than 4 strokes are...but 2 strokes can be finicky and require mechanical knowledge to get them to run at their top potential (speaking about chinese 2 strokes for motorized bikes that is).