The most common type of scanner, the kind you're likely to find in your local computer store, is a flatbed scanner. It has a glass bed usually a bit bigger than A4 paper size (or U.S. Letter if you live in the U.S.! :-) and most of the common models are optimized for typical office correspondence. One of these may cost anything from under $100 to $400, depending on its features, or you can pick them up cheaper second-hand. You use this by placing the paper or book face-down flat onto the glass, and scanning from there. This is the kind of scanner most commonly used by PG volunteers.
Some stores will call sheetfed scanners a different category. These are flatbed scanners with Automatic Document Feed (ADF), but they are fundamentally the same machine, and the ADF sheetfeeder unit may often be bought as an accessory to the flatbed scanner. Recently, a few sheetfed scanners have appeared that are very small, without a full flatbed, just a narrow strip that the paper rolls through. Avoid these for PG work; you often need to be able to scan the book flat.
Hand scanners, as their name implies, are much smaller, and typically very cheap, or even thrown in free. You use these by holding them in your hand and running them along the text like a brush. These are really not intended for PG work; you need a very steady hand movement to get them to scan a page of text into a readable image, and they shouldn't be considered as an option for a 400-page book--scanning and OCR is tough enough without that!
You can think of production scanners as industrial-strength flatbed scanners. The basic mechanisms are the same, but a production scanner will certainly have ADF (sheetfeeder), more features and speed, and be rated for very high volume scanning. Production scanners are used by publishers, businesses with high-volume paper processing needs, and print shops. This last is useful, because you may be able to get some scanning done by a print shop. It can't hurt to ask. If you're thinking about buying one of these babies (and who among us hasn't? :-), be sure you have $2000 or more to spend.
Drum scanners are mostly used by publishers for professional, high-quality artwork. The paper is placed on the surface of a drum that rotates past a fixed scanning head. The drum can be very large. Because the sensors don't have to move, the electronics and optics can be of higher quality, and produce very accurate, high-definition images. They are exactly what you would want for making professional quality scans of old movie posters, but they're expensive, and not very useful for scanning War and Peace to OCR.
Planetary scanners are a different breed to all the others. They are really not scanners at all, but a very high-end digital camera on a stand. You place the book face-up with the pages open, with the camera looking straight down on it. It takes a picture, and passes it on to the connected computer. Planetary scanners are ideal for old, fragile, valuable books that can't be exposed to the stress of normal scanning. They typically come supplied with specialized software, sometimes even their own dedicated computer, and they are very, very expensive--$20,000+.