There a four main types of scanners all virtually do the same thing by scanning a image so that its viewable on the computer its connected to.
It's most likely that these will be the scanners sitting on your desk at home. They are very bulky and dont always produce a clear image.
The concept of this device is virtually the same as a Flatbed scanner, though the paper is fed into the machine. These scanners are much smaller compared to the Flatbed. These are also made for home and office use.
Again its really the same concept of a Flatbed scanner, Though the user has to move the scanner manually over the text, The result usually ends up been very bad quality.
These scanners are very expensive and are usually used in the printing industry, Although there quality is very good compared to other scanners on the market.
Overall the flatbed scanner is the best in my opinion, its fast, easy and usually of a good enough quality. However it does depend on the use required, the Handheld scanner and Drum scanner are very good if they are required for special use.