A daisy wheel is a wheel used in a daisy wheel printer.
The daisy wheel itself consisted of a wheel of spokes, each spoke containing a printable character. When a letter was being printed, the wheel would rotate to the proper character and "bang" it out onto the paper (via the standard ribbon printing process), giving it an imprinted look and feel.
The daisy wheel printing method was first patented in 1942 by General Electric. It was first commercialized by Diablo (later bought out by Xerox) and reached its zenith in the late 1960s. It could print up to 80 characters per second, making it ideal for early 110 and 240 baud modems. Of course, the daisy wheel was soon circumvented by dot matrix printers, and are seeing as an archaic relic today.
The big advantage to the daisy wheel over other printing methods available at the time was that you could change the font simply by changing out the daisy wheel. However, they are extremely loud (louder than a dot matrix!) due to the "bang" nature of the printed character, and of course, compared to even the cheapest of modern-day inkjet and laser printers, they are too slow to even merit consideration.