In Dota 2 there are two opposing factions, the Radiant (bottom-left) and the Dire (top-right).
Radiant base logo
Dire base logo
Old Dire base logo
In WC3 DotA, the Radiant was called "the Sentinel" and the Dire was called "the Scourge", after the Night Elf Sentinels and the Undead Scourge in the Warcraft universe.
Heroes were divided by factions, and in the WC3 DotA exclusive "Normal Mode", you can only select Heroes from the faction you are playing as. This organization has been scrapped in Dota 2, however it is still referenced in the hero selection screen (for easier location for veteran players).
Arc Warden mentions the names "Diruulth and Radinthul", hinting that these are the "real" names of the entities locked within the faction's Ancients.