The two branches are divided mainly chronologically, rather than geographically:
Scythian - archaic version; mainly during classic antiquity
Sarmatian
Sometimes, the Scythian and Sarmatian languages are combined into one name: Scytho-Sarmatian languages.
Schythians migrated from Central Asia toward Eastern Europe, occupying today's Southern Russia and Ukraine. They dissapeared from history after the Hunnish invasion of the 5th century and most people speaking Scythian were probably assimilated by the Turkish/Tatar and Slavic people. However, part of the Scythians moved toward the Caucaus and modern Ossetic was derived from their language.
No written text in Scythian was ever found, but we know several toponyms, tribal names and personal names from the Greek inscriptions found at the Greek colonies on the Schythian shore of the Black Sea and we can also analize modern Ossetian for meanings of the words known from Greek texts.
Also, many toponyms and hydronyms of the Russian and Ukrainian steppe are believed to be of Scythian origin. For example, the name of the river Don derives from the Scythian *don, meaning water or river (reconstructed from the modern Ossetic).
External link
Scythian language (in Spanish) (http://www.geocities.com/linguaeimperii/Iranian/scythian_es.html)