There are blades that need to be sharpened, for example knives, axes, and chisels.
(note that there are a wide range of edges that need to be sharpened, including: straight razors, saw teeth, machinist cutters, various axes (adze, froe), and various bladed weapons)
- books that are highly recommended
- Wikipedia articles:
There are some relatively expensive jigs if you want to get a precise angle on your knives.
There are also a few cheap alternatives: