KOMA-Script is a family of documentclasses (scrbook scrartcl scrreprt scrlettr2), which allow you to customize your documents more than the standard classes (book article report letter) do.

LaTeX3 is a new programming language which is not directly useful for creating documents. It is meant to be the new way to write packages and classes. It is also a work in progress.

KOMA-Script is a family of documentclasses (scrbook scrartcl scrreprt scrlettr2), which allow you to customize your documents more than the standard classes (book article report letter) do.

LaTeX3 is right now mainly a new programming language which is not directly useful for creating documents. It is meant to be the new way to write packages and classes.

In the future LaTeX3 aims to be a complete replacement to LaTeX2e, with perhaps an entirely new command set to write documents, and also a ton of internal improvements to how things are handled. This is as for now a work in progress.