RuneScape Classic Wiki
The main RuneScape Wiki has an article on: Helmet
The Old School RuneScape Wiki also has an article on Helmet.

A helmet is a piece of armour worn on the head. Helmets generally fall into two categories, medium helmets and large helmets, large helmets providing the higher armour bonuses of the two and being more popular among players.

All helmets except the dragon medium helmet and Khazard Helmet can be made using the Smithing skill: medium helmets require one bar, large helmets require two. Peksa's Helmet Shop stocks helmets up to and including the Large Adamantite Helmet. The dragon medium helmet can only be obtained through monster drops, and the Khazard Helmet is obtained as part of the Fight Arena quest.