Knurling is a diamond-shaped criss-cross or similar pattern cut / rolled into metal.

The purpose of knurling is to aid with grip. It is commonly used on triggers / hammers / bolt handles / tool handles / etc.

Some call cartridge case / bullet cannelures knurl(s)(ing).

Footnotes / Sources:

“Glossary ⇒.” Sporting Arms and Ammunition Manufacturers’ Institute. Ed. SAAMI’s Technical Committee. Web. 15-18 Jan. 2011.