These pages are a part of the Slavic Interest Group, a volunteer-run project with the aim of putting together links to as much information about a particular areas’ history prior to 1600, we can find.
The project webpage describes it as:
The Slavic Knowledge/Slavianskoe Znan’stvo Project is intended to provide an easy means of gaining knowledge about the countries that SIG covers. Each page, while maintained by different people, is organized using similar subject categories to aid research.
This is the Baltic section, covering the modern-day states of Estonia, Latvia and Lithuania.

Map of the Modern-day Baltic States, from Wikipedia.
As these pages are divided into the modern territories, decisions about under which heading the various baltic tribes would be grouped, the maps from Virtual Livonia and Wikimedia were compared.
The Latvia pages will then also cover the Curonian, Latgalian, Livonian, Semigallian, and Selonian tribes.
The Lithuanian pages will cover the Galindian, Lithuanian, Prussian, Scalvian and Yotvingian tribes.
Estonia, in contrast, only the Estonians and the occasional Finn.