Letters are used as a way to communicate with others via the written word. They are then often taken from one person to another via a courier or some sort.
Letters have been used since at least the days of the Precursor race that lived in Karath-din and are still in common usage.
Letters can be written on a range of materials. The Precursors used a material which was hard wearing and many of their letters still exist in readable condition many hundreds of years later.[1][2]
In the days of Garrett, letters appeared to be written on some form of paper papyrus[3]. Some are written and put inside envelopes in order to protect the contents from unwanted eyes, through this doesn't always work.[4]