A picker (like in the pics above) takes only the cotton lint off the plant, leaving the boll, leaves, and stems behind. A stripper, on the other hand, takes every boll off the plant, along with considerable trash. A stripper works kind of like a corn head in reverse. There are two rollers (not sure on the technical term) inside each row unit that turn away from each other. They grab the plant and flip the bolls off, where they land in an auger. The auger carries the bolls to a saw cleaner (it's like a bunch of fine toothed circular saw blades), which clean a lot of the trash out of the lint before it's blown into the basket. Stripping cotton is also a once-over operation, but sometimes a picker can be ran across a field twice to catch bolls that opened later.
Cotton can be described in three ways, depending on how tightly the lint is held in the boll. Picker cotton is called open boll. The lint in open boll cotton is held the loosest. A hard rain can knock the lint off the plant. Next is storm resistant. The lint is held a little tighter, and it can be harvested with either a picker or a stripper. Finally, there's storm proof, which can only be harvested with a stripper. The lint in stripper cotton is held too tightly for the spindles on the picker to pull it out.
Picker cotton is also the highest quality (longest fibers), while stripper cotton is the lowest quality (shortest fibers). There are many other factors in grading cotton, but generally picker cotton is better for most fabrics.
This is what stripper cotton looks like. Notice all the sticks and pieces of the boll. All this has to be cleaned out at the gin.
And that concludes today's lesson.