Hardened saw teeth vs. resharpenable
Saws have either hardened or resharpenable teeth.
These are teeth which have not been hardened and so are designed to be resharpened when they become blunt.
If your saw has resharpenable teeth, there will usually be information on the product packaging or the saw itself that indicates this. Many resharpenable saws have a picture of a file and some teeth printed on the side of the blade.
Traditionally, almost all saws had teeth which could be resharpened.
Nowadays, the introduction of hardened teeth has changed things, and some users claim that as a result, saw sharpening has become somewhat of a lost art.
Hardened teeth (also called 'hardpoint' teeth)
These are teeth which have undergone a special hardening process. Although hardened teeth stay sharper for longer, when they eventually do become blunt, the saw must be thrown away and replaced.
Some manufacturers harden the entire blade, including the teeth. More commonly, only the teeth are hardened, and will often be blue or black in colour.
Because they have been specially hardened, these teeth are too tough to sharpen with a regular file. You may be able to purchase a diamond file to shape them, but this is likely to be expensive and inconvenient.
Sharpening teeth involves filing their edges to create a sharp point. The more you sharpen teeth, the further into the blade you are filing. As a result, if you try to sharpen a saw with hardened teeth, you will simply file away the hardened part and be left with the blade, which is likely to be made of a softer metal, one that will then become blunt again, extremely quickly.