Division into non-permanent (can be wiped off with water) and permanent OHP-Markers.

Permanent OHP-Markers:

• alcohol-based ink.
• lightfast and cannot be wiped off. 
• pens with a black barrel.

Non-permanent OHP-Markers:

• water-based ink. (They require the admixture of preservatives in order to prevent the growth of micro-organisms.
• not as lightfast as permanent inks and therefore not suited for long-term documentation.
• Can be wiped off with a moist cloth.
• pens with a grey barrel.