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.
In the days of Garrett, letters appeared to be written on some form of paper papyrus. Some are written and put inside envelopes in order to protect the contents from unwanted eyes, through this doesn't always work.