KOMA-Script is a bundle of classes and packages. The classes
scrlttr2 are extended replacements for the standard classes
letter, providing a lot more features for customization.
The KOMA-Script bundle contains packages which may also by used together with other classes, e.g.
scrlayer-scrpage as alternative to
fancyhdr for customizing header and footer.
Support for KOMA-Script is also available online (german only).